feat(android): MutationQueue PLAYLIST_APPEND + QUARANTINE_FLAG kinds

Extends the offline mutation queue with the two new write kinds the
TrackActions menu produces — appending a track to a playlist, and
flagging a track for quarantine. Replayer re-fires with idempotent
server semantics; payload data classes mirror Flutter's wire shapes.
Also adds PlaylistsApi.appendTracks since the replayer needs the
Retrofit surface in the same change set.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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

@@ -2,16 +2,17 @@ package com.fabledsword.minstrel.api.endpoints
 
 import com.fabledsword.minstrel.models.wire.PlaylistDetailWire
 import com.fabledsword.minstrel.models.wire.PlaylistsListWire
+import kotlinx.serialization.SerialName
+import kotlinx.serialization.Serializable
+import retrofit2.http.Body
 import retrofit2.http.GET
+import retrofit2.http.POST
 import retrofit2.http.Path
 import retrofit2.http.Query
 
 /**
  * Retrofit interface for `/api/playlists`. Mirrors
  * `flutter_client/lib/api/endpoints/playlists.dart`.
- *
- * Read-only for now — playlist creation, track-append, and the system
- * shuffle/refresh endpoints come in with the mutation-queue phase.
  */
 interface PlaylistsApi {
     /**
@@ -25,4 +26,25 @@ interface PlaylistsApi {
 
     @GET("api/playlists/{id}")
     suspend fun get(@Path("id") id: String): PlaylistDetailWire
+
+    /**
+     * POST /api/playlists/{id}/tracks — append the given track IDs to
+     * the owner's playlist. Server is idempotent at the (playlist_id,
+     * track_id) pair level; re-firing the same payload yields no
+     * duplicates.
+     */
+    @POST("api/playlists/{id}/tracks")
+    suspend fun appendTracks(
+        @Path("id") playlistId: String,
+        @Body body: AppendTracksRequest,
+    )
 }
+
+/**
+ * POST /api/playlists/{id}/tracks body. Server expects snake_case
+ * `track_ids` per the Flutter wire shape.
+ */
+@Serializable
+data class AppendTracksRequest(
+    @SerialName("track_ids") val trackIds: List<String>,
+)

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

@@ -17,6 +17,8 @@ object MutationKind {
     const val LIKE_TOGGLE: String = "like_toggle"
     const val REQUEST_CREATE: String = "request_create"
     const val QUARANTINE_UNFLAG: String = "quarantine_unflag"
+    const val PLAYLIST_APPEND: String = "playlist_append"
+    const val QUARANTINE_FLAG: String = "quarantine_flag"
 }
 
 /**
@@ -96,6 +98,33 @@ class MutationQueue @Inject constructor(
             ),
         ),
     )
+
+    suspend fun enqueuePlaylistAppend(
+        playlistId: String,
+        trackIds: List<String>,
+    ): Long = dao.insert(
+        CachedMutationEntity(
+            kind = MutationKind.PLAYLIST_APPEND,
+            payload = json.encodeToString(
+                PlaylistAppendPayload.serializer(),
+                PlaylistAppendPayload(playlistId, trackIds),
+            ),
+        ),
+    )
+
+    suspend fun enqueueQuarantineFlag(
+        trackId: String,
+        reason: String,
+        notes: String,
+    ): Long = dao.insert(
+        CachedMutationEntity(
+            kind = MutationKind.QUARANTINE_FLAG,
+            payload = json.encodeToString(
+                QuarantineFlagPayload.serializer(),
+                QuarantineFlagPayload(trackId, reason, notes),
+            ),
+        ),
+    )
 }
 
 /**
@@ -105,3 +134,28 @@ class MutationQueue @Inject constructor(
  */
 @Serializable
 data class QuarantineUnflagPayload(val trackId: String)
+
+/**
+ * Persisted payload for `MutationKind.PLAYLIST_APPEND`. Mirrors the
+ * Flutter wire shape: list of track IDs to append to a specific
+ * playlist. Server is idempotent at the (playlist_id, track_id) pair
+ * level — re-firing the same payload yields no duplicates.
+ */
+@Serializable
+data class PlaylistAppendPayload(
+    val playlistId: String,
+    val trackIds: List<String>,
+)
+
+/**
+ * Persisted payload for `MutationKind.QUARANTINE_FLAG`. The replayer
+ * re-fires `POST /api/quarantine` with this body when the original
+ * call failed; server-side, re-flagging an already-flagged (user,
+ * track) pair is idempotent.
+ */
+@Serializable
+data class QuarantineFlagPayload(
+    val trackId: String,
+    val reason: String,
+    val notes: String,
+)

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

@@ -1,7 +1,10 @@
 package com.fabledsword.minstrel.cache.mutations
 
+import com.fabledsword.minstrel.api.endpoints.AppendTracksRequest
 import com.fabledsword.minstrel.api.endpoints.DiscoverApi
+import com.fabledsword.minstrel.api.endpoints.FlagRequest
 import com.fabledsword.minstrel.api.endpoints.LikesApi
+import com.fabledsword.minstrel.api.endpoints.PlaylistsApi
 import com.fabledsword.minstrel.api.endpoints.QuarantineApi
 import com.fabledsword.minstrel.auth.AuthStore
 import com.fabledsword.minstrel.cache.db.dao.CachedMutationDao
@@ -57,6 +60,7 @@ class MutationReplayer @Inject constructor(
     private val likesApi: LikesApi = retrofit.create()
     private val discoverApi: DiscoverApi = retrofit.create()
     private val quarantineApi: QuarantineApi = retrofit.create()
+    private val playlistsApi: PlaylistsApi = retrofit.create()
 
     private val mutex = Mutex()
 
@@ -103,6 +107,8 @@ class MutationReplayer @Inject constructor(
         MutationKind.LIKE_TOGGLE -> dispatchLikeToggle(row.payload)
         MutationKind.REQUEST_CREATE -> dispatchRequestCreate(row.payload)
         MutationKind.QUARANTINE_UNFLAG -> dispatchQuarantineUnflag(row.payload)
+        MutationKind.PLAYLIST_APPEND -> dispatchPlaylistAppend(row.payload)
+        MutationKind.QUARANTINE_FLAG -> dispatchQuarantineFlag(row.payload)
         else -> {
             // Unknown kind — drop the row by claiming success so a
             // stale schema entry can't wedge the queue forever.
@@ -176,4 +182,39 @@ class MutationReplayer @Inject constructor(
             false
         }
     }
+
+    private suspend fun dispatchPlaylistAppend(payload: String): Boolean {
+        val decoded = json.decodeFromString(PlaylistAppendPayload.serializer(), payload)
+        return try {
+            playlistsApi.appendTracks(
+                playlistId = decoded.playlistId,
+                body = AppendTracksRequest(trackIds = decoded.trackIds),
+            )
+            true
+        } catch (
+            @Suppress("TooGenericExceptionCaught", "SwallowedException") e: Throwable,
+        ) {
+            // Intentional swallow: row stays queued; next drain pass retries.
+            false
+        }
+    }
+
+    private suspend fun dispatchQuarantineFlag(payload: String): Boolean {
+        val decoded = json.decodeFromString(QuarantineFlagPayload.serializer(), payload)
+        return try {
+            quarantineApi.flag(
+                FlagRequest(
+                    trackId = decoded.trackId,
+                    reason = decoded.reason,
+                    notes = decoded.notes,
+                ),
+            )
+            true
+        } catch (
+            @Suppress("TooGenericExceptionCaught", "SwallowedException") e: Throwable,
+        ) {
+            // Intentional swallow: row stays queued; next drain pass retries.
+            false
+        }
+    }
 }