feat: per-user volume leveling preference, synced across devices (M464 #4998)

Mode (off, auto, track, album), target (-18, -16, -14 LUFS) and boost
(within headroom, or fully with a limiter), stored per user on the server
so the web player, the Android app and casts apply the same one.

- Server: user_normalization_prefs (migration 0067), GET/PUT
  /api/me/normalization; a whole-body PUT, validated, last write wins.
- Web: Settings > Playback > Volume leveling. Saves at once, restores the
  old choice if the save fails, and caches the value for the player.
- Android: Settings card. The device keeps a copy for offline playback
  (Room v9 with an explicit migration, so the upgrade wipes nothing).
  Writes are offline-first: shown at once, PUT best effort, queued on
  failure (NORMALIZATION_SET, collapsed to the newest). A refresh never
  overwrites a change still queued.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

android/app/src/main/java/com/fabledsword/minstrel/api/endpoints/MeApi.kt

@@ -3,6 +3,7 @@ package com.fabledsword.minstrel.api.endpoints
 import com.fabledsword.minstrel.models.wire.ListenBrainzStatusWire
 import com.fabledsword.minstrel.models.wire.MyProfileWire
 import com.fabledsword.minstrel.models.wire.SystemPlaylistsStatusWire
+import com.fabledsword.minstrel.settings.data.NormalizationPrefs
 import kotlinx.serialization.SerialName
 import kotlinx.serialization.Serializable
 import retrofit2.http.Body
@@ -59,6 +60,14 @@ interface MeApi {
      */
     @PUT("api/me/listenbrainz")
     suspend fun setListenBrainz(@Body body: ListenBrainzPutBody): ListenBrainzStatusWire
+
+    /** The caller's loudness-normalization preference, or the defaults if never set. */
+    @GET("api/me/normalization")
+    suspend fun getNormalization(): NormalizationPrefs
+
+    /** Replaces the whole preference; returns what the server stored. */
+    @PUT("api/me/normalization")
+    suspend fun putNormalization(@Body body: NormalizationPrefs): NormalizationPrefs
 }
 
 /**

android/app/src/main/java/com/fabledsword/minstrel/auth/AuthStore.kt

@@ -4,6 +4,7 @@ import com.fabledsword.minstrel.cache.audiocache.CacheSettings
 import com.fabledsword.minstrel.cache.db.dao.AuthSessionDao
 import com.fabledsword.minstrel.cache.db.entities.AuthSessionEntity
 import com.fabledsword.minstrel.di.ApplicationScope
+import com.fabledsword.minstrel.settings.data.NormalizationPrefs
 import kotlinx.coroutines.CoroutineScope
 import kotlinx.coroutines.Deferred
 import kotlinx.coroutines.async
@@ -77,6 +78,9 @@ class AuthStore @Inject constructor(
     private val diagnosticsOptOutState = MutableStateFlow(false)
     val diagnosticsOptOut: StateFlow<Boolean> = diagnosticsOptOutState.asStateFlow()
 
+    private val normalizationState = MutableStateFlow(NormalizationPrefs.DEFAULT)
+    val normalization: StateFlow<NormalizationPrefs> = normalizationState.asStateFlow()
+
     private val json = Json { ignoreUnknownKeys = true }
 
     // Serialises every cookie persist with the one-time hydration, so a
@@ -99,6 +103,7 @@ class AuthStore @Inject constructor(
                 clientIdState.value = row?.clientId
                 cacheSettingsState.value = decodeCacheSettings(row?.cacheSettingsJson)
                 diagnosticsOptOutState.value = row?.diagnosticsOptOut ?: false
+                normalizationState.value = decodeNormalization(row?.normalizationJson)
             }
         }
     }
@@ -110,6 +115,13 @@ class AuthStore @Inject constructor(
         }.getOrDefault(CacheSettings.DEFAULT)
     }
 
+    private fun decodeNormalization(raw: String?): NormalizationPrefs {
+        if (raw.isNullOrEmpty()) return NormalizationPrefs.DEFAULT
+        return runCatching {
+            json.decodeFromString(NormalizationPrefs.serializer(), raw)
+        }.getOrDefault(NormalizationPrefs.DEFAULT)
+    }
+
     /**
      * Suspends until the stored session cookie has been loaded into
      * [sessionCookie], or [HYDRATION_DEADLINE_MS] passes (rule 156: a wedged
@@ -187,6 +199,12 @@ class AuthStore @Inject constructor(
         scope.launch { persistDiagnosticsOptOut(value) }
     }
 
+    fun setNormalization(value: NormalizationPrefs) {
+        normalizationState.value = value
+        val encoded = json.encodeToString(NormalizationPrefs.serializer(), value)
+        scope.launch { persistNormalization(encoded) }
+    }
+
     private suspend fun persistLegacyCookie(value: String?) {
         if (dao.get() == null) {
             dao.upsert(currentEntity().copy(sessionCookie = value))
@@ -243,6 +261,14 @@ class AuthStore @Inject constructor(
         }
     }
 
+    private suspend fun persistNormalization(json: String) {
+        if (dao.get() == null) {
+            dao.upsert(currentEntity().copy(normalizationJson = json))
+        } else {
+            dao.setNormalizationJson(json)
+        }
+    }
+
     private fun currentEntity(): AuthSessionEntity = AuthSessionEntity(
         id = ROW_ID,
         // Never copied into the row: the cookie lives in the vault, and
@@ -257,6 +283,10 @@ class AuthStore @Inject constructor(
             cacheSettingsState.value,
         ),
         diagnosticsOptOut = diagnosticsOptOutState.value,
+        normalizationJson = json.encodeToString(
+            NormalizationPrefs.serializer(),
+            normalizationState.value,
+        ),
     )
 
     companion object {

android/app/src/main/java/com/fabledsword/minstrel/cache/db/AppDatabase.kt

@@ -3,6 +3,8 @@ package com.fabledsword.minstrel.cache.db
 import androidx.room.Database
 import androidx.room.RoomDatabase
 import androidx.room.TypeConverters
+import androidx.room.migration.Migration
+import androidx.sqlite.db.SupportSQLiteDatabase
 import com.fabledsword.minstrel.cache.db.dao.AudioCacheIndexDao
 import com.fabledsword.minstrel.cache.db.dao.AuthSessionDao
 import com.fabledsword.minstrel.cache.db.dao.CachedAlbumDao
@@ -65,13 +67,18 @@ import com.fabledsword.minstrel.cache.db.entities.SyncMetadataEntity
         AuthSessionEntity::class,
         DiagnosticEventEntity::class,
     ],
+    // v9: + auth_session.normalizationJson, the loudness-normalization
+    // preference (M464 #4998). The first schema step with an explicit
+    // Migration (MIGRATION_8_9): a destructive rebuild would also wipe this
+    // row — the server address and theme — and the queued offline writes,
+    // which is too much to lose for one added column.
     // v8: + cached_tracks.missing, the server's missing-file mark (#2704),
     // so cache-first surfaces stop offering files that cannot stream.
     // v7: + diagnostic_events table (M9) and the diagnosticsOptOut column
     // on auth_session. Pre-v1 destructive fallback rebuilds on mismatch —
     // which is exactly right here: the next sync refills every row with the
     // new column populated, so there is nothing to migrate by hand.
-    version = 8,
+    version = 9,
     exportSchema = true,
 )
 @TypeConverters(MinstrelTypeConverters::class)
@@ -92,3 +99,10 @@ abstract class AppDatabase : RoomDatabase() {
     abstract fun authSessionDao(): AuthSessionDao
     abstract fun diagnosticEventDao(): DiagnosticEventDao
 }
+
+/** v8 → v9: add the nullable normalization preference column (#4998). */
+val MIGRATION_8_9: Migration = object : Migration(8, 9) {
+    override fun migrate(db: SupportSQLiteDatabase) {
+        db.execSQL("ALTER TABLE auth_session ADD COLUMN normalizationJson TEXT")
+    }
+}

android/app/src/main/java/com/fabledsword/minstrel/cache/db/DatabaseModule.kt

@@ -37,6 +37,7 @@ object DatabaseModule {
             // launch, so users lose only the unsynced mutation queue
             // (acceptable while we're iterating). Replace with explicit
             // Migration entries before the first tagged release.
+            .addMigrations(MIGRATION_8_9)
             .fallbackToDestructiveMigration(dropAllTables = true)
             .build()
 

android/app/src/main/java/com/fabledsword/minstrel/cache/db/dao/AuthSessionDao.kt

@@ -46,4 +46,8 @@ interface AuthSessionDao {
     /** Partial update: change only the per-device diagnostics opt-out. */
     @Query("UPDATE auth_session SET diagnosticsOptOut = :optOut WHERE id = 0")
     suspend fun setDiagnosticsOptOut(optOut: Boolean)
+
+    /** Partial update: change only the serialized normalization preference. */
+    @Query("UPDATE auth_session SET normalizationJson = :json WHERE id = 0")
+    suspend fun setNormalizationJson(json: String?)
 }

android/app/src/main/java/com/fabledsword/minstrel/cache/db/dao/CachedMutationDao.kt

@@ -31,4 +31,8 @@ interface CachedMutationDao {
 
     @Query("DELETE FROM cached_mutations")
     suspend fun clear()
+
+    /** Whether a write of [kind] is still waiting to be replayed. */
+    @Query("SELECT EXISTS(SELECT 1 FROM cached_mutations WHERE kind = :kind)")
+    suspend fun hasPending(kind: String): Boolean
 }

android/app/src/main/java/com/fabledsword/minstrel/cache/db/entities/AuthSessionEntity.kt

@@ -43,4 +43,10 @@ data class AuthSessionEntity(
      * choice lives here. Default false = honor the account flag.
      */
     val diagnosticsOptOut: Boolean = false,
+    /**
+     * JSON-encoded NormalizationPrefs (settings/data), the last value seen
+     * from the server or set here (M464 #4998). Null = never fetched; the
+     * defaults apply. Kept so offline playback still levels.
+     */
+    val normalizationJson: String? = null,
 )

android/app/src/main/java/com/fabledsword/minstrel/cache/mutations/MutationQueue.kt

@@ -2,6 +2,7 @@ package com.fabledsword.minstrel.cache.mutations
 
 import com.fabledsword.minstrel.cache.db.dao.CachedMutationDao
 import com.fabledsword.minstrel.cache.db.entities.CachedMutationEntity
+import com.fabledsword.minstrel.settings.data.NormalizationPrefs
 import kotlinx.coroutines.channels.BufferOverflow
 import kotlinx.coroutines.flow.MutableSharedFlow
 import kotlinx.coroutines.flow.SharedFlow
@@ -41,6 +42,11 @@ object MutationKind {
     // an undo collapses to the latest intent instead of replaying as two
     // opposed calls whose order decides the outcome.
     const val SUGGESTION_SNOOZE_TOGGLE: String = "suggestion_snooze_toggle"
+
+    // M464 #4998 loudness-normalization preference. The payload is the whole
+    // preference, a target state like the toggles above, so queued changes
+    // collapse to the last one and an older one can never be replayed last.
+    const val NORMALIZATION_SET: String = "normalization_set"
 }
 
 /**
@@ -177,6 +183,12 @@ class MutationQueue @Inject constructor(
         ),
     )
 
+    /** Queues the user's whole normalization preference for replay. */
+    suspend fun enqueueNormalizationSet(prefs: NormalizationPrefs): Long = insertUserDriven(
+        MutationKind.NORMALIZATION_SET,
+        json.encodeToString(NormalizationPrefs.serializer(), prefs),
+    )
+
     suspend fun enqueueRequestCancel(requestId: String): Long = insertUserDriven(
         MutationKind.REQUEST_CANCEL,
         json.encodeToString(

android/app/src/main/java/com/fabledsword/minstrel/cache/mutations/MutationReplayer.kt

@@ -7,6 +7,7 @@ import com.fabledsword.minstrel.api.endpoints.DiscoverApi
 import com.fabledsword.minstrel.api.endpoints.EventsApi
 import com.fabledsword.minstrel.api.endpoints.FlagRequest
 import com.fabledsword.minstrel.api.endpoints.LikesApi
+import com.fabledsword.minstrel.api.endpoints.MeApi
 import com.fabledsword.minstrel.api.endpoints.PlaybackErrorReportRequest
 import com.fabledsword.minstrel.api.endpoints.PlaybackErrorsApi
 import com.fabledsword.minstrel.api.endpoints.PlaylistsApi
@@ -22,6 +23,7 @@ import com.fabledsword.minstrel.cache.db.dao.CachedMutationDao
 import com.fabledsword.minstrel.cache.db.entities.CachedMutationEntity
 import com.fabledsword.minstrel.di.ApplicationScope
 import com.fabledsword.minstrel.likes.data.LikesRepository
+import com.fabledsword.minstrel.settings.data.NormalizationPrefs
 import com.fabledsword.minstrel.models.wire.CreateRequestBody
 import kotlinx.coroutines.CoroutineScope
 import kotlinx.coroutines.flow.distinctUntilChanged
@@ -79,6 +81,7 @@ class MutationReplayer @Inject constructor(
     private val eventsApi: EventsApi = retrofit.create()
     private val requestsApi: RequestsApi = retrofit.create()
     private val playbackErrorsApi: PlaybackErrorsApi = retrofit.create()
+    private val meApi: MeApi = retrofit.create()
 
     private val mutex = Mutex()
 
@@ -166,6 +169,7 @@ class MutationReplayer @Inject constructor(
         MutationKind.REQUEST_CANCEL -> dispatchRequestCancel(row.payload)
         MutationKind.PLAYBACK_ERROR_REPORT -> dispatchPlaybackErrorReport(row.payload)
         MutationKind.SUGGESTION_SNOOZE_TOGGLE -> dispatchSuggestionSnoozeToggle(row.payload)
+        MutationKind.NORMALIZATION_SET -> dispatchNormalizationSet(row.payload)
         // Unknown kind — drop so a stale schema entry can't wedge the queue.
         else -> Outcome.DROP
     }
@@ -279,6 +283,16 @@ class MutationReplayer @Inject constructor(
         return Outcome.SENT
     }
 
+    /**
+     * Sends the queued normalization preference. The device already shows
+     * it, so the server's echo is not written back: a change made since the
+     * row was queued would be a newer row, and the collapse keeps only that.
+     */
+    private suspend fun dispatchNormalizationSet(payload: String): Outcome {
+        meApi.putNormalization(json.decodeFromString(NormalizationPrefs.serializer(), payload))
+        return Outcome.SENT
+    }
+
     private suspend fun dispatchPlaybackErrorReport(payload: String): Outcome {
         val decoded = json.decodeFromString(PlaybackErrorReportPayload.serializer(), payload)
         playbackErrorsApi.report(
@@ -303,7 +317,8 @@ class MutationReplayer @Inject constructor(
 /**
  * Row ids of desired-state toggles superseded by a later toggle for the same
  * entity. Applies to every kind whose payload encodes a TARGET state rather
- * than an action — like-toggles and suggestion snoozes (#2374) — because
+ * than an action — like-toggles, suggestion snoozes (#2374) and the
+ * normalization preference (#4998) — because
  * replaying a stale one last would invert the final state.
  *
  * Top-level and pure so it can be unit-tested without standing up a Retrofit
@@ -340,5 +355,10 @@ private fun toggleKeyOf(row: CachedMutationEntity, json: Json): String? = when (
         json.decodeFromString(SuggestionSnoozeTogglePayload.serializer(), row.payload)
     }.getOrNull()?.let { "${row.kind}:${it.mbid}" }
 
+    // One preference per user, so every normalization row shares one key.
+    MutationKind.NORMALIZATION_SET -> runCatching {
+        json.decodeFromString(NormalizationPrefs.serializer(), row.payload)
+    }.getOrNull()?.let { row.kind }
+
     else -> null
 }

android/app/src/main/java/com/fabledsword/minstrel/settings/data/NormalizationPrefs.kt

@@ -0,0 +1,55 @@
+package com.fabledsword.minstrel.settings.data
+
+import kotlinx.serialization.SerialName
+import kotlinx.serialization.Serializable
+
+/**
+ * The user's loudness-normalization preference (M464 #4998). The server
+ * holds it, so this device, the web player and casts all level the same
+ * way; a copy is kept on the device so offline playback still applies it.
+ * Field names and values match `GET/PUT /api/me/normalization`.
+ *
+ * No constructor defaults, on purpose: the app's Json leaves out any field
+ * equal to its default (encodeDefaults is off), and the server refuses a PUT
+ * missing a field. Without defaults every field is always sent.
+ */
+@Serializable
+data class NormalizationPrefs(
+    val mode: NormalizationMode,
+    @SerialName("target_lufs") val targetLufs: Int,
+    val boost: NormalizationBoost,
+) {
+    companion object {
+        /** The targets the server accepts, quietest first. */
+        val TARGETS: List<Int> = listOf(-18, -16, -14)
+
+        /** Must match library.DefaultNormalizationPrefs on the server. */
+        val DEFAULT: NormalizationPrefs = NormalizationPrefs(
+            mode = NormalizationMode.AUTO,
+            targetLufs = -18,
+            boost = NormalizationBoost.HEADROOM,
+        )
+    }
+}
+
+@Serializable
+enum class NormalizationMode {
+    /** Tracks play at their mastered volume. */
+    @SerialName("off") OFF,
+
+    /** Album gain while an album plays in order, track gain otherwise. */
+    @SerialName("auto") AUTO,
+
+    @SerialName("track") TRACK,
+
+    @SerialName("album") ALBUM,
+}
+
+@Serializable
+enum class NormalizationBoost {
+    /** Quiet tracks are raised only as far as their true peak allows. */
+    @SerialName("headroom") HEADROOM,
+
+    /** Quiet tracks are raised all the way and a limiter holds the peaks. */
+    @SerialName("limiter") LIMITER,
+}

android/app/src/main/java/com/fabledsword/minstrel/settings/data/NormalizationRepository.kt

@@ -0,0 +1,67 @@
+package com.fabledsword.minstrel.settings.data
+
+import com.fabledsword.minstrel.api.endpoints.MeApi
+import com.fabledsword.minstrel.auth.AuthStore
+import com.fabledsword.minstrel.cache.db.dao.CachedMutationDao
+import com.fabledsword.minstrel.cache.mutations.MutationKind
+import com.fabledsword.minstrel.cache.mutations.MutationQueue
+import kotlinx.coroutines.CancellationException
+import kotlinx.coroutines.flow.StateFlow
+import retrofit2.Retrofit
+import retrofit2.create
+import timber.log.Timber
+import javax.inject.Inject
+import javax.inject.Singleton
+
+/**
+ * The user's loudness-normalization preference (M464 #4998). The device's
+ * copy lives in [AuthStore] so playback reads it offline; the server's copy
+ * is what other devices and casts use.
+ *
+ * Writes follow rule 100: the device shows the change at once, the PUT is
+ * best effort, and a failed PUT is queued for the MutationReplayer.
+ */
+@Singleton
+class NormalizationRepository @Inject constructor(
+    retrofit: Retrofit,
+    private val authStore: AuthStore,
+    private val mutationQueue: MutationQueue,
+    private val mutationDao: CachedMutationDao,
+) {
+    private val api: MeApi = retrofit.create()
+
+    val prefs: StateFlow<NormalizationPrefs> = authStore.normalization
+
+    /**
+     * Takes the server's value, unless a change made here is still queued:
+     * the server has not seen it yet, and taking its older value would undo
+     * the change on screen until the replay landed. Throws on a network
+     * failure; the device's copy then stands.
+     */
+    suspend fun refresh() {
+        val server = api.getNormalization()
+        if (mutationDao.hasPending(MutationKind.NORMALIZATION_SET)) return
+        authStore.setNormalization(server)
+    }
+
+    suspend fun set(next: NormalizationPrefs) {
+        authStore.setNormalization(next)
+        // An earlier change still queued would be replayed after this PUT
+        // and undo it. Queue behind it instead: the replayer sends only the
+        // newest queued preference.
+        if (mutationDao.hasPending(MutationKind.NORMALIZATION_SET)) {
+            mutationQueue.enqueueNormalizationSet(next)
+            return
+        }
+        try {
+            api.putNormalization(next)
+        } catch (e: CancellationException) {
+            throw e
+        } catch (
+            @Suppress("TooGenericExceptionCaught") e: Throwable,
+        ) {
+            Timber.i(e, "normalization: PUT failed; queued for replay")
+            mutationQueue.enqueueNormalizationSet(next)
+        }
+    }
+}

android/app/src/main/java/com/fabledsword/minstrel/settings/ui/NormalizationCard.kt

@@ -0,0 +1,143 @@
+package com.fabledsword.minstrel.settings.ui
+
+import androidx.compose.foundation.layout.Arrangement
+import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.Row
+import androidx.compose.foundation.layout.Spacer
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.foundation.layout.padding
+import androidx.compose.foundation.layout.size
+import androidx.compose.material3.ElevatedCard
+import androidx.compose.material3.ExperimentalMaterial3Api
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.SegmentedButton
+import androidx.compose.material3.SegmentedButtonDefaults
+import androidx.compose.material3.SingleChoiceSegmentedButtonRow
+import androidx.compose.material3.Switch
+import androidx.compose.material3.Text
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.getValue
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.unit.dp
+import androidx.hilt.navigation.compose.hiltViewModel
+import androidx.lifecycle.compose.collectAsStateWithLifecycle
+import com.fabledsword.minstrel.settings.data.NormalizationBoost
+import com.fabledsword.minstrel.settings.data.NormalizationMode
+import com.fabledsword.minstrel.settings.data.NormalizationPrefs
+
+/**
+ * Volume leveling (M464 #4998). The choice is stored on the server, so the
+ * web player and casts follow it too.
+ */
+@Composable
+fun NormalizationCard(viewModel: NormalizationViewModel = hiltViewModel()) {
+    val prefs by viewModel.prefs.collectAsStateWithLifecycle()
+    NormalizationCardContent(prefs = prefs, onChange = viewModel::update)
+}
+
+@OptIn(ExperimentalMaterial3Api::class)
+@Composable
+internal fun NormalizationCardContent(
+    prefs: NormalizationPrefs,
+    onChange: (NormalizationPrefs) -> Unit,
+) {
+    ElevatedCard(modifier = Modifier.fillMaxWidth()) {
+        Column(
+            modifier = Modifier.padding(16.dp),
+            verticalArrangement = Arrangement.spacedBy(8.dp),
+        ) {
+            Text(
+                text = "Volume leveling",
+                style = MaterialTheme.typography.titleMedium,
+                color = MaterialTheme.colorScheme.onSurface,
+            )
+            Hint(modeHint(prefs.mode))
+            val modes = NormalizationMode.entries
+            SingleChoiceSegmentedButtonRow(modifier = Modifier.fillMaxWidth()) {
+                modes.forEachIndexed { index, mode ->
+                    SegmentedButton(
+                        selected = mode == prefs.mode,
+                        onClick = { onChange(prefs.copy(mode = mode)) },
+                        shape = SegmentedButtonDefaults.itemShape(
+                            index = index,
+                            count = modes.size,
+                        ),
+                    ) { Text(modeLabel(mode)) }
+                }
+            }
+            if (prefs.mode != NormalizationMode.OFF) {
+                TargetRow(prefs = prefs, onChange = onChange)
+                BoostRow(prefs = prefs, onChange = onChange)
+            }
+        }
+    }
+}
+
+@OptIn(ExperimentalMaterial3Api::class)
+@Composable
+private fun TargetRow(prefs: NormalizationPrefs, onChange: (NormalizationPrefs) -> Unit) {
+    Text(
+        text = "Target loudness",
+        style = MaterialTheme.typography.bodyLarge,
+        color = MaterialTheme.colorScheme.onSurface,
+    )
+    val targets = NormalizationPrefs.TARGETS
+    SingleChoiceSegmentedButtonRow(modifier = Modifier.fillMaxWidth()) {
+        targets.forEachIndexed { index, lufs ->
+            SegmentedButton(
+                selected = lufs == prefs.targetLufs,
+                onClick = { onChange(prefs.copy(targetLufs = lufs)) },
+                shape = SegmentedButtonDefaults.itemShape(
+                    index = index,
+                    count = targets.size,
+                ),
+            ) { Text("$lufs LUFS") }
+        }
+    }
+}
+
+@Composable
+private fun BoostRow(prefs: NormalizationPrefs, onChange: (NormalizationPrefs) -> Unit) {
+    Row(verticalAlignment = Alignment.CenterVertically) {
+        Column(modifier = Modifier.weight(1f)) {
+            Text(
+                text = "Boost quiet tracks fully",
+                style = MaterialTheme.typography.bodyLarge,
+                color = MaterialTheme.colorScheme.onSurface,
+            )
+            Hint("A limiter catches the peaks.")
+        }
+        Spacer(Modifier.size(12.dp))
+        Switch(
+            checked = prefs.boost == NormalizationBoost.LIMITER,
+            onCheckedChange = { on ->
+                val boost = if (on) NormalizationBoost.LIMITER else NormalizationBoost.HEADROOM
+                onChange(prefs.copy(boost = boost))
+            },
+        )
+    }
+}
+
+@Composable
+private fun Hint(text: String) {
+    Text(
+        text = text,
+        style = MaterialTheme.typography.bodySmall,
+        color = MaterialTheme.colorScheme.onSurfaceVariant,
+    )
+}
+
+private fun modeLabel(mode: NormalizationMode): String = when (mode) {
+    NormalizationMode.OFF -> "Off"
+    NormalizationMode.AUTO -> "Auto"
+    NormalizationMode.TRACK -> "Track"
+    NormalizationMode.ALBUM -> "Album"
+}
+
+private fun modeHint(mode: NormalizationMode): String = when (mode) {
+    NormalizationMode.OFF -> "Tracks play at their mastered volume."
+    NormalizationMode.AUTO -> "Album gain for whole albums, track gain otherwise."
+    NormalizationMode.TRACK -> "Every track at the same loudness."
+    NormalizationMode.ALBUM -> "Albums keep their own quiet and loud tracks."
+}

android/app/src/main/java/com/fabledsword/minstrel/settings/ui/NormalizationViewModel.kt

@@ -0,0 +1,44 @@
+package com.fabledsword.minstrel.settings.ui
+
+import androidx.lifecycle.ViewModel
+import androidx.lifecycle.viewModelScope
+import com.fabledsword.minstrel.settings.data.NormalizationPrefs
+import com.fabledsword.minstrel.settings.data.NormalizationRepository
+import dagger.hilt.android.lifecycle.HiltViewModel
+import kotlinx.coroutines.CancellationException
+import kotlinx.coroutines.flow.StateFlow
+import kotlinx.coroutines.launch
+import timber.log.Timber
+import javax.inject.Inject
+
+/**
+ * Backs the Volume leveling card. Shows the device's copy straight away and
+ * refreshes it from the server on open, so a change made on the web shows
+ * here too.
+ */
+@HiltViewModel
+class NormalizationViewModel @Inject constructor(
+    private val repository: NormalizationRepository,
+) : ViewModel() {
+
+    val prefs: StateFlow<NormalizationPrefs> = repository.prefs
+
+    init {
+        viewModelScope.launch {
+            try {
+                repository.refresh()
+            } catch (e: CancellationException) {
+                throw e
+            } catch (
+                @Suppress("TooGenericExceptionCaught") e: Throwable,
+            ) {
+                // Offline: the device's copy stands.
+                Timber.i(e, "normalization: refresh failed")
+            }
+        }
+    }
+
+    fun update(next: NormalizationPrefs) {
+        viewModelScope.launch { repository.set(next) }
+    }
+}

android/app/src/main/java/com/fabledsword/minstrel/settings/ui/SettingsScreen.kt

@@ -161,6 +161,7 @@ private fun SettingsList(
                 onToggle = onToggleDiagnostics,
             )
         }
+        NormalizationCard()
         AppearanceCard(themeMode = themeMode, onPick = onPickTheme)
         StorageCard()
         AboutCard()

android/app/src/test/java/com/fabledsword/minstrel/cache/mutations/SupersededToggleIdsTest.kt

@@ -1,6 +1,8 @@
 package com.fabledsword.minstrel.cache.mutations
 
 import com.fabledsword.minstrel.cache.db.entities.CachedMutationEntity
+import com.fabledsword.minstrel.settings.data.NormalizationMode
+import com.fabledsword.minstrel.settings.data.NormalizationPrefs
 import kotlinx.serialization.json.Json
 import org.junit.jupiter.api.Test
 import kotlin.test.assertEquals
@@ -129,4 +131,26 @@ class SupersededToggleIdsTest {
     fun `an empty queue collapses nothing`() {
         assertTrue(supersededToggleIds(emptyList(), json).isEmpty())
     }
+
+    private fun normalizationRow(id: Long, mode: NormalizationMode) = CachedMutationEntity(
+        id = id,
+        kind = MutationKind.NORMALIZATION_SET,
+        payload = json.encodeToString(
+            NormalizationPrefs.serializer(),
+            NormalizationPrefs.DEFAULT.copy(mode = mode),
+        ),
+    )
+
+    // There is one preference per user, so any two queued changes to it
+    // collapse, and only the newest is sent (#4998).
+    @Test
+    fun `only the newest queued normalization change survives`() {
+        val rows = listOf(
+            normalizationRow(1, NormalizationMode.TRACK),
+            snoozeRow(2, "mb-a", desiredSnoozed = true),
+            normalizationRow(3, NormalizationMode.OFF),
+            normalizationRow(4, NormalizationMode.ALBUM),
+        )
+        assertEquals(setOf(1L, 3L), supersededToggleIds(rows, json))
+    }
 }

android/app/src/test/java/com/fabledsword/minstrel/settings/data/NormalizationRepositoryTest.kt

@@ -0,0 +1,90 @@
+package com.fabledsword.minstrel.settings.data
+
+import com.fabledsword.minstrel.api.endpoints.MeApi
+import com.fabledsword.minstrel.auth.AuthStore
+import com.fabledsword.minstrel.cache.db.dao.CachedMutationDao
+import com.fabledsword.minstrel.cache.mutations.MutationKind
+import com.fabledsword.minstrel.cache.mutations.MutationQueue
+import io.mockk.coEvery
+import io.mockk.coVerify
+import io.mockk.every
+import io.mockk.mockk
+import io.mockk.verify
+import kotlinx.coroutines.flow.MutableStateFlow
+import kotlinx.coroutines.test.runTest
+import org.junit.jupiter.api.Test
+import retrofit2.Retrofit
+import java.io.IOException
+
+/**
+ * The normalization preference is written offline-first (rule 100): the
+ * device shows the change at once, and a change the server has not seen
+ * yet is never overwritten or reordered.
+ */
+class NormalizationRepositoryTest {
+    private val api: MeApi = mockk()
+    private val authStore: AuthStore = mockk(relaxed = true) {
+        every { normalization } returns MutableStateFlow(NormalizationPrefs.DEFAULT)
+    }
+    private val queue: MutationQueue = mockk(relaxed = true)
+    private val dao: CachedMutationDao = mockk()
+    private val retrofit: Retrofit = mockk {
+        every { create(MeApi::class.java) } returns api
+    }
+    private val repo = NormalizationRepository(retrofit, authStore, queue, dao)
+
+    private val album = NormalizationPrefs.DEFAULT.copy(mode = NormalizationMode.ALBUM)
+
+    @Test
+    fun `a change is shown at once and sent`() = runTest {
+        coEvery { dao.hasPending(MutationKind.NORMALIZATION_SET) } returns false
+        coEvery { api.putNormalization(album) } returns album
+
+        repo.set(album)
+
+        verify { authStore.setNormalization(album) }
+        coVerify { api.putNormalization(album) }
+        coVerify(exactly = 0) { queue.enqueueNormalizationSet(any()) }
+    }
+
+    @Test
+    fun `a change that cannot reach the server is queued`() = runTest {
+        coEvery { dao.hasPending(MutationKind.NORMALIZATION_SET) } returns false
+        coEvery { api.putNormalization(album) } throws IOException("offline")
+
+        repo.set(album)
+
+        verify { authStore.setNormalization(album) }
+        coVerify { queue.enqueueNormalizationSet(album) }
+    }
+
+    @Test
+    fun `a change behind a queued one queues too, so the older one cannot land last`() = runTest {
+        coEvery { dao.hasPending(MutationKind.NORMALIZATION_SET) } returns true
+
+        repo.set(album)
+
+        coVerify(exactly = 0) { api.putNormalization(any()) }
+        coVerify { queue.enqueueNormalizationSet(album) }
+    }
+
+    @Test
+    fun `refresh takes the server value`() = runTest {
+        coEvery { api.getNormalization() } returns album
+        coEvery { dao.hasPending(MutationKind.NORMALIZATION_SET) } returns false
+
+        repo.refresh()
+
+        verify { authStore.setNormalization(album) }
+    }
+
+    @Test
+    fun `refresh leaves a queued change on screen`() = runTest {
+        coEvery { api.getNormalization() } returns NormalizationPrefs.DEFAULT
+        coEvery { dao.hasPending(MutationKind.NORMALIZATION_SET) } returns true
+
+        repo.refresh()
+
+        verify(exactly = 0) { authStore.setNormalization(any()) }
+    }
+}

internal/api/api.go

@@ -109,6 +109,8 @@ func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playev
 			authed.Put("/me/password", h.handleChangePassword)
 			authed.Put("/me/profile", h.handleUpdateMyProfile)
 			authed.Put("/me/timezone", h.handlePutTimezone)
+			authed.Get("/me/normalization", h.handleGetMyNormalization)
+			authed.Put("/me/normalization", h.handlePutMyNormalization)
 			authed.Post("/me/api-token", h.handleRegenerateMyAPIToken)
 			authed.Get("/me/subsonic-password", h.handleGetMySubsonicPassword)
 			authed.Post("/me/subsonic-password", h.handleGenerateMySubsonicPassword)

internal/api/me_normalization.go

@@ -0,0 +1,51 @@
+package api
+
+import (
+	"encoding/json"
+	"errors"
+	"net/http"
+
+	"git.fabledsword.com/bvandeusen/minstrel/internal/apierror"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/library"
+)
+
+// GET /api/me/normalization returns the caller's loudness-normalization
+// preference, or the defaults when they have never set one (M464 #4998).
+func (h *handlers) handleGetMyNormalization(w http.ResponseWriter, r *http.Request) {
+	caller, ok := requireUser(w, r)
+	if !ok {
+		return
+	}
+	p, err := library.LoadNormalizationPrefs(r.Context(), dbq.New(h.pool), caller.ID)
+	if err != nil {
+		writeErrWithLog(w, h.logger, "normalization: load", apierror.InternalMsg("lookup failed", err))
+		return
+	}
+	writeJSON(w, http.StatusOK, p)
+}
+
+// PUT /api/me/normalization replaces the caller's preference with the whole
+// body. Every field is required, so a client replaying a queued change
+// offline sends the full state it showed the user, and the last write wins.
+func (h *handlers) handlePutMyNormalization(w http.ResponseWriter, r *http.Request) {
+	caller, ok := requireUser(w, r)
+	if !ok {
+		return
+	}
+	var body library.NormalizationPrefs
+	if err := json.NewDecoder(r.Body).Decode(&body); err != nil {
+		writeErr(w, apierror.BadRequest("bad_body", "invalid JSON body"))
+		return
+	}
+	p, err := library.SaveNormalizationPrefs(r.Context(), dbq.New(h.pool), caller.ID, body)
+	if errors.Is(err, library.ErrNormalizationPrefsInvalid) {
+		writeErr(w, apierror.BadRequest("invalid_normalization", err.Error()))
+		return
+	}
+	if err != nil {
+		writeErrWithLog(w, h.logger, "normalization: save", apierror.InternalMsg("save failed", err))
+		return
+	}
+	writeJSON(w, http.StatusOK, p)
+}

internal/api/me_normalization_test.go

@@ -0,0 +1,61 @@
+package api
+
+import (
+	"bytes"
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/go-chi/chi/v5"
+
+	"git.fabledsword.com/bvandeusen/minstrel/internal/library"
+)
+
+func TestMyNormalization_GetPut(t *testing.T) {
+	h, pool := testHandlers(t)
+	user := seedUser(t, pool, "norm1", "pw", false)
+	r := chi.NewRouter()
+	r.Get("/api/me/normalization", h.handleGetMyNormalization)
+	r.Put("/api/me/normalization", h.handlePutMyNormalization)
+	do := func(method, body string) (int, library.NormalizationPrefs) {
+		t.Helper()
+		req := withUser(httptest.NewRequest(method, "/api/me/normalization", bytes.NewBufferString(body)), user)
+		rec := httptest.NewRecorder()
+		r.ServeHTTP(rec, req)
+		var p library.NormalizationPrefs
+		if rec.Code == http.StatusOK {
+			if err := json.Unmarshal(rec.Body.Bytes(), &p); err != nil {
+				t.Fatalf("decode %s: %v", rec.Body.String(), err)
+			}
+		}
+		return rec.Code, p
+	}
+
+	if code, p := do(http.MethodGet, ""); code != http.StatusOK || p != library.DefaultNormalizationPrefs {
+		t.Fatalf("GET before any save = %d %+v, want 200 with the defaults", code, p)
+	}
+
+	want := library.NormalizationPrefs{Mode: "album", TargetLUFS: -16, Boost: "limiter"}
+	if code, p := do(http.MethodPut, `{"mode":"album","target_lufs":-16,"boost":"limiter"}`); code != http.StatusOK || p != want {
+		t.Fatalf("PUT = %d %+v, want 200 %+v", code, p, want)
+	}
+	if code, p := do(http.MethodGet, ""); code != http.StatusOK || p != want {
+		t.Errorf("GET after PUT = %d %+v, want %+v", code, p, want)
+	}
+
+	// Bad values and partial bodies are refused, and the stored prefs stand.
+	for _, body := range []string{
+		`{"mode":"loud","target_lufs":-16,"boost":"limiter"}`,
+		`{"mode":"album","target_lufs":-23,"boost":"limiter"}`,
+		`{"mode":"album","target_lufs":-16}`,
+		`not json`,
+	} {
+		if code, _ := do(http.MethodPut, body); code != http.StatusBadRequest {
+			t.Errorf("PUT %s = %d, want 400", body, code)
+		}
+	}
+	if _, p := do(http.MethodGet, ""); p != want {
+		t.Errorf("a refused PUT changed the prefs to %+v", p)
+	}
+}

internal/db/dbq/models.go

@@ -784,6 +784,14 @@ type UserInvite struct {
 	RedeemedBy pgtype.UUID
 }
 
+type UserNormalizationPref struct {
+	UserID     pgtype.UUID
+	Mode       string
+	TargetLufs int16
+	Boost      string
+	UpdatedAt  pgtype.Timestamptz
+}
+
 type YouMightLikeAlbum struct {
 	UserID  pgtype.UUID
 	AlbumID pgtype.UUID

internal/db/dbq/normalization_prefs.sql.go

@@ -0,0 +1,81 @@
+// Code generated by sqlc. DO NOT EDIT.
+// versions:
+//   sqlc v1.31.1
+// source: normalization_prefs.sql
+
+package dbq
+
+import (
+	"context"
+
+	"github.com/jackc/pgx/v5/pgtype"
+)
+
+const getNormalizationPrefs = `-- name: GetNormalizationPrefs :one
+SELECT mode, target_lufs, boost, updated_at
+  FROM user_normalization_prefs
+ WHERE user_id = $1
+`
+
+type GetNormalizationPrefsRow struct {
+	Mode       string
+	TargetLufs int16
+	Boost      string
+	UpdatedAt  pgtype.Timestamptz
+}
+
+func (q *Queries) GetNormalizationPrefs(ctx context.Context, userID pgtype.UUID) (GetNormalizationPrefsRow, error) {
+	row := q.db.QueryRow(ctx, getNormalizationPrefs, userID)
+	var i GetNormalizationPrefsRow
+	err := row.Scan(
+		&i.Mode,
+		&i.TargetLufs,
+		&i.Boost,
+		&i.UpdatedAt,
+	)
+	return i, err
+}
+
+const upsertNormalizationPrefs = `-- name: UpsertNormalizationPrefs :one
+INSERT INTO user_normalization_prefs (user_id, mode, target_lufs, boost)
+VALUES ($1, $2, $3, $4)
+ON CONFLICT (user_id) DO UPDATE SET
+    mode        = EXCLUDED.mode,
+    target_lufs = EXCLUDED.target_lufs,
+    boost       = EXCLUDED.boost,
+    updated_at  = now()
+RETURNING mode, target_lufs, boost, updated_at
+`
+
+type UpsertNormalizationPrefsParams struct {
+	UserID     pgtype.UUID
+	Mode       string
+	TargetLufs int16
+	Boost      string
+}
+
+type UpsertNormalizationPrefsRow struct {
+	Mode       string
+	TargetLufs int16
+	Boost      string
+	UpdatedAt  pgtype.Timestamptz
+}
+
+// Whole-row write from either client; the CHECKs in migration 0067 back the
+// handler's own validation.
+func (q *Queries) UpsertNormalizationPrefs(ctx context.Context, arg UpsertNormalizationPrefsParams) (UpsertNormalizationPrefsRow, error) {
+	row := q.db.QueryRow(ctx, upsertNormalizationPrefs,
+		arg.UserID,
+		arg.Mode,
+		arg.TargetLufs,
+		arg.Boost,
+	)
+	var i UpsertNormalizationPrefsRow
+	err := row.Scan(
+		&i.Mode,
+		&i.TargetLufs,
+		&i.Boost,
+		&i.UpdatedAt,
+	)
+	return i, err
+}

internal/db/migrations/0067_user_normalization_prefs.down.sql

@@ -0,0 +1 @@
+DROP TABLE IF EXISTS user_normalization_prefs;

internal/db/migrations/0067_user_normalization_prefs.up.sql

@@ -0,0 +1,22 @@
+-- Each user's loudness-normalization preference (M464 #4998). It is kept on
+-- the server, not per device, so the web player, the Android app and the
+-- leveled stream for UPnP/Sonos all apply the same one.
+--
+-- mode:   off, auto (album gain while an album plays in order, otherwise
+--         track gain), track or album.
+-- target: the loudness tracks are leveled to, in LUFS.
+-- boost:  headroom raises quiet tracks only as far as their true peak
+--         allows; limiter raises them all the way and limits the peaks.
+--
+-- No row means the defaults, which are the column defaults here and
+-- library.DefaultNormalizationPrefs in Go (a test holds them equal).
+CREATE TABLE user_normalization_prefs (
+    user_id     uuid PRIMARY KEY REFERENCES users(id) ON DELETE CASCADE,
+    mode        text NOT NULL DEFAULT 'auto'
+                CHECK (mode IN ('off', 'auto', 'track', 'album')),
+    target_lufs smallint NOT NULL DEFAULT -18
+                CHECK (target_lufs IN (-18, -16, -14)),
+    boost       text NOT NULL DEFAULT 'headroom'
+                CHECK (boost IN ('headroom', 'limiter')),
+    updated_at  timestamptz NOT NULL DEFAULT now()
+);

internal/db/queries/normalization_prefs.sql

@@ -0,0 +1,16 @@
+-- name: GetNormalizationPrefs :one
+SELECT mode, target_lufs, boost, updated_at
+  FROM user_normalization_prefs
+ WHERE user_id = $1;
+
+-- name: UpsertNormalizationPrefs :one
+-- Whole-row write from either client; the CHECKs in migration 0067 back the
+-- handler's own validation.
+INSERT INTO user_normalization_prefs (user_id, mode, target_lufs, boost)
+VALUES (sqlc.arg(user_id), sqlc.arg(mode), sqlc.arg(target_lufs), sqlc.arg(boost))
+ON CONFLICT (user_id) DO UPDATE SET
+    mode        = EXCLUDED.mode,
+    target_lufs = EXCLUDED.target_lufs,
+    boost       = EXCLUDED.boost,
+    updated_at  = now()
+RETURNING mode, target_lufs, boost, updated_at;

internal/dbtest/reset.go

@@ -93,6 +93,7 @@ var dataTables = []string{
 	"track_fingerprints",       // M400
 	"track_loudness",           // M464
 	"album_loudness",           // M464
+	"user_normalization_prefs", // M464
 	"tracks",
 	"albums",
 	"artists",

internal/library/normalization_prefs.go

@@ -0,0 +1,84 @@
+package library
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"slices"
+
+	"github.com/jackc/pgx/v5"
+	"github.com/jackc/pgx/v5/pgtype"
+
+	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+)
+
+// NormalizationPrefs is one user's loudness-normalization preference (M464
+// #4998). It lives on the server so every player applies the same one: the
+// web player and the Android app read it, and the leveled stream for
+// UPnP/Sonos (#5001) renders with it.
+type NormalizationPrefs struct {
+	// Mode is off, auto, track or album. Auto uses album gain while an album
+	// plays in order and track gain otherwise.
+	Mode string `json:"mode"`
+	// TargetLUFS is the loudness tracks are leveled to.
+	TargetLUFS int16 `json:"target_lufs"`
+	// Boost is how a track quieter than the target is raised: headroom stops
+	// at its true peak, limiter goes all the way and limits the peaks.
+	Boost string `json:"boost"`
+}
+
+// The accepted values. Migration 0067's CHECKs hold the same lists.
+var (
+	NormalizationModes   = []string{"off", "auto", "track", "album"}
+	NormalizationTargets = []int16{-18, -16, -14}
+	NormalizationBoosts  = []string{"headroom", "limiter"}
+)
+
+// DefaultNormalizationPrefs applies to a user who has never chosen. It must
+// equal migration 0067's column defaults; a test holds them together.
+var DefaultNormalizationPrefs = NormalizationPrefs{Mode: "auto", TargetLUFS: -18, Boost: "headroom"}
+
+// ErrNormalizationPrefsInvalid is returned for a value outside the lists.
+var ErrNormalizationPrefsInvalid = errors.New("normalization preference out of range")
+
+// Validate reports the first field holding a value the server does not
+// accept.
+func (p NormalizationPrefs) Validate() error {
+	switch {
+	case !slices.Contains(NormalizationModes, p.Mode):
+		return fmt.Errorf("%w: mode %q", ErrNormalizationPrefsInvalid, p.Mode)
+	case !slices.Contains(NormalizationTargets, p.TargetLUFS):
+		return fmt.Errorf("%w: target_lufs %d", ErrNormalizationPrefsInvalid, p.TargetLUFS)
+	case !slices.Contains(NormalizationBoosts, p.Boost):
+		return fmt.Errorf("%w: boost %q", ErrNormalizationPrefsInvalid, p.Boost)
+	}
+	return nil
+}
+
+// LoadNormalizationPrefs returns the user's preference, or the defaults if
+// they have never set one.
+func LoadNormalizationPrefs(ctx context.Context, q *dbq.Queries, userID pgtype.UUID) (NormalizationPrefs, error) {
+	row, err := q.GetNormalizationPrefs(ctx, userID)
+	if errors.Is(err, pgx.ErrNoRows) {
+		return DefaultNormalizationPrefs, nil
+	}
+	if err != nil {
+		return DefaultNormalizationPrefs, err
+	}
+	return NormalizationPrefs{Mode: row.Mode, TargetLUFS: row.TargetLufs, Boost: row.Boost}, nil
+}
+
+// SaveNormalizationPrefs validates and stores the user's preference,
+// returning what was stored.
+func SaveNormalizationPrefs(ctx context.Context, q *dbq.Queries, userID pgtype.UUID, p NormalizationPrefs) (NormalizationPrefs, error) {
+	if err := p.Validate(); err != nil {
+		return p, err
+	}
+	row, err := q.UpsertNormalizationPrefs(ctx, dbq.UpsertNormalizationPrefsParams{
+		UserID: userID, Mode: p.Mode, TargetLufs: p.TargetLUFS, Boost: p.Boost,
+	})
+	if err != nil {
+		return p, err
+	}
+	return NormalizationPrefs{Mode: row.Mode, TargetLUFS: row.TargetLufs, Boost: row.Boost}, nil
+}

internal/library/normalization_prefs_test.go

@@ -0,0 +1,69 @@
+package library
+
+import (
+	"context"
+	"errors"
+	"testing"
+
+	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/dbtest"
+)
+
+func TestNormalizationPrefs_Validate(t *testing.T) {
+	if err := DefaultNormalizationPrefs.Validate(); err != nil {
+		t.Fatalf("the defaults are invalid: %v", err)
+	}
+	for _, p := range []NormalizationPrefs{
+		{Mode: "loud", TargetLUFS: -18, Boost: "headroom"},
+		{Mode: "auto", TargetLUFS: -23, Boost: "headroom"},
+		{Mode: "auto", TargetLUFS: -18, Boost: "max"},
+		// A field left out of a PUT body decodes as its zero value.
+		{Mode: "auto", Boost: "headroom"},
+	} {
+		if err := p.Validate(); !errors.Is(err, ErrNormalizationPrefsInvalid) {
+			t.Errorf("%+v: err = %v, want ErrNormalizationPrefsInvalid", p, err)
+		}
+	}
+}
+
+// A user with no row gets the Go defaults, and a stored row reads back the
+// migration's defaults for any column the insert left out; the two must
+// agree, or a user would hear a different level after their first save.
+func TestNormalizationPrefs_DefaultsAndRoundTrip_Integration(t *testing.T) {
+	pool := newPool(t)
+	ctx := context.Background()
+	q := dbq.New(pool)
+	user, err := q.CreateUser(ctx, dbq.CreateUserParams{
+		Username: dbtest.TestUserPrefix + "norm-prefs", PasswordHash: "x", ApiTokenHash: "norm-prefs-token",
+	})
+	if err != nil {
+		t.Fatalf("user: %v", err)
+	}
+
+	got, err := LoadNormalizationPrefs(ctx, q, user.ID)
+	if err != nil || got != DefaultNormalizationPrefs {
+		t.Fatalf("unset prefs = %+v, %v; want the defaults", got, err)
+	}
+
+	if _, err := pool.Exec(ctx, "INSERT INTO user_normalization_prefs (user_id) VALUES ($1)", user.ID); err != nil {
+		t.Fatalf("insert defaults row: %v", err)
+	}
+	got, err = LoadNormalizationPrefs(ctx, q, user.ID)
+	if err != nil || got != DefaultNormalizationPrefs {
+		t.Errorf("migration defaults = %+v, Go defaults = %+v (err %v)", got, DefaultNormalizationPrefs, err)
+	}
+
+	want := NormalizationPrefs{Mode: "track", TargetLUFS: -14, Boost: "limiter"}
+	if saved, err := SaveNormalizationPrefs(ctx, q, user.ID, want); err != nil || saved != want {
+		t.Fatalf("save = %+v, %v; want %+v", saved, err, want)
+	}
+	if got, _ := LoadNormalizationPrefs(ctx, q, user.ID); got != want {
+		t.Errorf("after save = %+v, want %+v", got, want)
+	}
+	if _, err := SaveNormalizationPrefs(ctx, q, user.ID, NormalizationPrefs{Mode: "x"}); !errors.Is(err, ErrNormalizationPrefsInvalid) {
+		t.Errorf("invalid save: err = %v", err)
+	}
+	if got, _ := LoadNormalizationPrefs(ctx, q, user.ID); got != want {
+		t.Errorf("an invalid save changed the stored prefs to %+v", got)
+	}
+}

web/src/lib/api/normalization.ts

@@ -0,0 +1,42 @@
+import { api } from './client';
+
+// Loudness normalization preference (M464 #4998). One per user, stored on
+// the server so the web player, the Android app and casts agree.
+export type NormalizationMode = 'off' | 'auto' | 'track' | 'album';
+export type NormalizationTarget = -18 | -16 | -14;
+export type NormalizationBoost = 'headroom' | 'limiter';
+
+export type NormalizationPrefs = {
+  mode: NormalizationMode;
+  target_lufs: NormalizationTarget;
+  boost: NormalizationBoost;
+};
+
+// Must match library.DefaultNormalizationPrefs on the server.
+export const DEFAULT_NORMALIZATION: NormalizationPrefs = {
+  mode: 'auto',
+  target_lufs: -18,
+  boost: 'headroom'
+};
+
+export const NORMALIZATION_MODES: NormalizationMode[] = ['off', 'auto', 'track', 'album'];
+export const NORMALIZATION_TARGETS: NormalizationTarget[] = [-18, -16, -14];
+
+export function isNormalizationPrefs(v: unknown): v is NormalizationPrefs {
+  if (typeof v !== 'object' || v === null) return false;
+  const p = v as Record<string, unknown>;
+  return (
+    NORMALIZATION_MODES.includes(p.mode as NormalizationMode) &&
+    NORMALIZATION_TARGETS.includes(p.target_lufs as NormalizationTarget) &&
+    (p.boost === 'headroom' || p.boost === 'limiter')
+  );
+}
+
+export function getNormalization(): Promise<NormalizationPrefs> {
+  return api.get<NormalizationPrefs>('/api/me/normalization');
+}
+
+// The whole preference is sent every time; the server keeps the last write.
+export function putNormalization(p: NormalizationPrefs): Promise<NormalizationPrefs> {
+  return api.put<NormalizationPrefs>('/api/me/normalization', p);
+}

web/src/lib/components/NormalizationSetting.svelte

@@ -0,0 +1,104 @@
+<script lang="ts">
+  import { onMount } from 'svelte';
+  import {
+    NORMALIZATION_MODES,
+    NORMALIZATION_TARGETS,
+    type NormalizationPrefs
+  } from '$lib/api/normalization';
+  import {
+    normalization,
+    loadNormalization,
+    setNormalization
+  } from '$lib/stores/normalization.svelte';
+  import { pushToast } from '$lib/stores/toast.svelte';
+
+  // Volume leveling (M464 #4998). The choice is the user's, stored on the
+  // server, so it follows them to the Android app and to casts.
+  onMount(() => {
+    void loadNormalization();
+  });
+
+  const modeLabels: Record<string, string> = {
+    off: 'Off',
+    auto: 'Auto',
+    track: 'Track',
+    album: 'Album'
+  };
+  const modeHints: Record<string, string> = {
+    off: 'Tracks play at their mastered volume.',
+    auto: 'Album gain for whole albums in order, track gain otherwise.',
+    track: 'Every track at the same loudness.',
+    album: 'Albums keep their own quiet and loud tracks.'
+  };
+
+  async function change(patch: Partial<NormalizationPrefs>): Promise<void> {
+    try {
+      await setNormalization({ ...normalization.value, ...patch });
+    } catch {
+      pushToast('Could not save volume leveling.', 'error');
+    }
+  }
+
+  const pill = (on: boolean) =>
+    `flex cursor-pointer items-center rounded border border-border px-3 py-1.5 text-sm ${
+      on ? 'bg-action-secondary text-action-fg' : 'text-text-primary hover:bg-surface-hover'
+    }`;
+</script>
+
+<div class="space-y-3">
+  <fieldset class="space-y-2">
+    <legend class="block text-sm text-text-secondary">Volume leveling</legend>
+    <div class="flex flex-wrap gap-2">
+      {#each NORMALIZATION_MODES as m (m)}
+        <label class={pill(normalization.value.mode === m)}>
+          <input
+            type="radio"
+            name="normalization-mode"
+            value={m}
+            checked={normalization.value.mode === m}
+            onchange={() => change({ mode: m })}
+            class="sr-only"
+          />
+          {modeLabels[m]}
+        </label>
+      {/each}
+    </div>
+    <p class="text-xs text-text-secondary">{modeHints[normalization.value.mode]}</p>
+  </fieldset>
+
+  {#if normalization.value.mode !== 'off'}
+    <fieldset class="space-y-2">
+      <legend class="block text-sm text-text-secondary">Target loudness</legend>
+      <div class="flex flex-wrap gap-2">
+        {#each NORMALIZATION_TARGETS as t (t)}
+          <label class={pill(normalization.value.target_lufs === t)}>
+            <input
+              type="radio"
+              name="normalization-target"
+              value={t}
+              checked={normalization.value.target_lufs === t}
+              onchange={() => change({ target_lufs: t })}
+              class="sr-only"
+            />
+            <span class="tabular-nums">{t} LUFS</span>
+          </label>
+        {/each}
+      </div>
+      <p class="text-xs text-text-secondary">Higher is louder; −18 leaves the most headroom.</p>
+    </fieldset>
+
+    <label class="flex cursor-pointer items-center justify-between gap-3">
+      <span>
+        <span class="block text-sm text-text-primary">Boost quiet tracks fully</span>
+        <span class="block text-xs text-text-secondary">A limiter catches the peaks.</span>
+      </span>
+      <input
+        type="checkbox"
+        checked={normalization.value.boost === 'limiter'}
+        onchange={(e) =>
+          change({ boost: (e.currentTarget as HTMLInputElement).checked ? 'limiter' : 'headroom' })}
+        class="h-4 w-4 accent-accent"
+      />
+    </label>
+  {/if}
+</div>

web/src/lib/components/NormalizationSetting.test.ts

@@ -0,0 +1,65 @@
+import { afterEach, describe, expect, test, vi } from 'vitest';
+import { render, screen, fireEvent, waitFor } from '@testing-library/svelte';
+
+vi.mock('$lib/api/normalization', async (importOriginal) => ({
+  ...(await importOriginal<typeof import('$lib/api/normalization')>()),
+  getNormalization: vi.fn(),
+  putNormalization: vi.fn()
+}));
+vi.mock('$lib/stores/toast.svelte', () => ({ pushToast: vi.fn() }));
+
+import NormalizationSetting from './NormalizationSetting.svelte';
+import { getNormalization, putNormalization } from '$lib/api/normalization';
+import { normalization } from '$lib/stores/normalization.svelte';
+import { pushToast } from '$lib/stores/toast.svelte';
+
+afterEach(() => {
+  vi.clearAllMocks();
+  localStorage.clear();
+});
+
+describe('NormalizationSetting', () => {
+  test('shows the server value and saves the whole preference on a change', async () => {
+    vi.mocked(getNormalization).mockResolvedValue({ mode: 'track', target_lufs: -16, boost: 'headroom' });
+    vi.mocked(putNormalization).mockImplementation(async (p) => p);
+    render(NormalizationSetting);
+
+    await waitFor(() => expect(screen.getByRole('radio', { name: 'Track' })).toBeChecked());
+    expect(screen.getByRole('radio', { name: '-16 LUFS' })).toBeChecked();
+
+    await fireEvent.click(screen.getByRole('radio', { name: 'Album' }));
+    await waitFor(() =>
+      expect(putNormalization).toHaveBeenCalledWith({ mode: 'album', target_lufs: -16, boost: 'headroom' })
+    );
+    await fireEvent.click(screen.getByRole('checkbox', { name: /boost quiet tracks/i }));
+    await waitFor(() =>
+      expect(putNormalization).toHaveBeenLastCalledWith({ mode: 'album', target_lufs: -16, boost: 'limiter' })
+    );
+    // The last value seen is cached for the player's first track of a session.
+    expect(JSON.parse(localStorage.getItem('minstrel:normalization') ?? 'null')).toEqual({
+      mode: 'album',
+      target_lufs: -16,
+      boost: 'limiter'
+    });
+  });
+
+  test('a failed save puts the previous choice back and says so', async () => {
+    vi.mocked(getNormalization).mockResolvedValue({ mode: 'auto', target_lufs: -18, boost: 'headroom' });
+    vi.mocked(putNormalization).mockRejectedValue(new Error('offline'));
+    render(NormalizationSetting);
+    await waitFor(() => expect(screen.getByRole('radio', { name: 'Auto' })).toBeChecked());
+
+    await fireEvent.click(screen.getByRole('radio', { name: 'Off' }));
+    await waitFor(() => expect(pushToast).toHaveBeenCalledWith('Could not save volume leveling.', 'error'));
+    expect(normalization.value.mode).toBe('auto');
+    expect(screen.getByRole('radio', { name: 'Auto' })).toBeChecked();
+  });
+
+  test('off hides the target and boost controls', async () => {
+    vi.mocked(getNormalization).mockResolvedValue({ mode: 'off', target_lufs: -18, boost: 'headroom' });
+    render(NormalizationSetting);
+    await waitFor(() => expect(screen.getByRole('radio', { name: 'Off' })).toBeChecked());
+    expect(screen.queryByRole('radio', { name: '-18 LUFS' })).toBeNull();
+    expect(screen.queryByRole('checkbox')).toBeNull();
+  });
+});

web/src/lib/stores/normalization.svelte.ts

@@ -0,0 +1,66 @@
+import * as storage from '$lib/util/safeLocalStorage';
+import {
+  DEFAULT_NORMALIZATION,
+  getNormalization,
+  isNormalizationPrefs,
+  putNormalization,
+  type NormalizationPrefs
+} from '$lib/api/normalization';
+
+// The signed-in user's normalization preference. The server holds it; this
+// keeps the last value seen in localStorage too, so the player levels the
+// first track of a session before the fetch returns.
+const STORAGE_KEY = 'minstrel:normalization';
+
+function readCached(): NormalizationPrefs {
+  try {
+    const v: unknown = JSON.parse(storage.read(STORAGE_KEY) ?? 'null');
+    return isNormalizationPrefs(v) ? v : DEFAULT_NORMALIZATION;
+  } catch {
+    return DEFAULT_NORMALIZATION;
+  }
+}
+
+let _prefs = $state<NormalizationPrefs>(readCached());
+// Bumped by every change made here. A load or a save that resolves after a
+// newer change must not overwrite it.
+let _generation = 0;
+
+export const normalization = {
+  get value(): NormalizationPrefs {
+    return _prefs;
+  }
+};
+
+function commit(p: NormalizationPrefs): void {
+  _prefs = p;
+  storage.write(STORAGE_KEY, JSON.stringify(p));
+}
+
+/** Refreshes from the server; on failure the cached value stands. */
+export async function loadNormalization(): Promise<void> {
+  const gen = _generation;
+  try {
+    const p = await getNormalization();
+    if (gen === _generation) commit(p);
+  } catch {
+    // Offline or signed out: keep what we had.
+  }
+}
+
+/**
+ * Applies [next] at once, then saves it. If the save fails the previous
+ * value is restored and the error rethrown for the caller to report.
+ */
+export async function setNormalization(next: NormalizationPrefs): Promise<void> {
+  const prev = _prefs;
+  const gen = ++_generation;
+  commit(next);
+  try {
+    const saved = await putNormalization(next);
+    if (gen === _generation) commit(saved);
+  } catch (err) {
+    if (gen === _generation) commit(prev);
+    throw err;
+  }
+}

web/src/routes/settings/+page.svelte

@@ -30,6 +30,7 @@
   import MobileAppDownload from '$lib/components/MobileAppDownload.svelte';
   import ServerVersion from '$lib/components/ServerVersion.svelte';
   import ActiveSessions from '$lib/components/ActiveSessions.svelte';
+  import NormalizationSetting from '$lib/components/NormalizationSetting.svelte';
 
   const queryClient = useQueryClient();
 
@@ -341,6 +342,8 @@
         0 = off · most albums sound best at 0.
       </p>
     </div>
+
+    <NormalizationSetting />
   </section>
 
   <section class="space-y-4 rounded border border-border bg-surface p-4">

web/src/routes/settings/Appearance.test.ts

@@ -25,6 +25,16 @@ vi.mock('$lib/api/listenbrainz', () => {
   };
 });
 
+// The Volume leveling setting loads on mount; keep it off the network.
+vi.mock('$lib/api/normalization', async (importOriginal) => {
+  const mod = await importOriginal<typeof import('$lib/api/normalization')>();
+  return {
+    ...mod,
+    getNormalization: vi.fn().mockResolvedValue(mod.DEFAULT_NORMALIZATION),
+    putNormalization: vi.fn()
+  };
+});
+
 vi.mock('$lib/api/metrics', () => ({
   createRecommendationMetricsQuery: () => ({
     subscribe: (run: (v: unknown) => void) => {

web/src/routes/settings/settings.test.ts

@@ -11,6 +11,16 @@ vi.mock('$lib/api/listenbrainz', () => ({
   setListenBrainzEnabled: vi.fn()
 }));
 
+// The Volume leveling setting loads on mount; keep it off the network.
+vi.mock('$lib/api/normalization', async (importOriginal) => {
+  const mod = await importOriginal<typeof import('$lib/api/normalization')>();
+  return {
+    ...mod,
+    getNormalization: vi.fn().mockResolvedValue(mod.DEFAULT_NORMALIZATION),
+    putNormalization: vi.fn()
+  };
+});
+
 vi.mock('$lib/api/me', () => ({
   updateProfile: vi.fn(),
   changePassword: vi.fn(),