feat(android): detect zero-duration tracks, fail fast, report to server

Operator hit a track that loaded with zero duration; player just sat
on it. Two things needed: skip the dead track immediately, and tell
the server so the admin inbox can surface the bad file.

PlayerController:
- Player.Listener.onPlaybackStateChanged(STATE_READY) now checks
  duration. If it's <= 0 or C.TIME_UNSET, fires a PlaybackErrorEvent
  with kind="zero_duration" and calls seekToNextMediaItem (or stop
  if it was the last item). Per-item evaluation guard keeps repeat
  STATE_READY events (post-seek, post-resume) from re-firing.
- onPlayerError now also surfaces a PlaybackErrorEvent with
  kind="load_failed" + the Media3 exception message as detail.
- playbackErrorEvents flow changes from Flow<String> (title only) to
  Flow<PlaybackErrorEvent> (track_id + kind + title + detail) so
  downstream consumers can both surface a snackbar AND POST to the
  admin inbox without duplicating event emission.

PlaybackErrorRepository (new):
- Wraps POST /api/playback-errors with the offline-first MutationQueue
  fallback per the standing rule for server writes.
- Reuses AuthStore.clientId for the client_id field — same UUID-per-
  install identifier the play-events reporter sends, so support can
  correlate playback errors with surrounding plays.

PlaybackErrorReporter:
- Consumes the new richer event shape. Fires the server report per
  event (no debounce — the admin inbox should capture every report,
  not a coalesced summary). Continues to debounce the user-facing
  snackbar in the 2s window so a burst doesn't spam toasts.

MutationQueue / MutationReplayer:
- Adds PLAYBACK_ERROR_REPORT kind + PlaybackErrorReportPayload +
  enqueuePlaybackErrorReport entry point + replayer dispatch case
  hitting the new PlaybackErrorsApi.

Web admin inbox + UI is the next commit.

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

@@ -0,0 +1,33 @@
+package com.fabledsword.minstrel.api.endpoints
+
+import retrofit2.http.Body
+import retrofit2.http.POST
+
+/**
+ * Retrofit interface for `POST /api/playback-errors`. Reports a
+ * client-detected playback failure (zero-duration, decode error, ...)
+ * so the admin inbox surfaces it.
+ *
+ * Failures during dispatch are reported via the MutationQueue path
+ * for offline replay (see [com.fabledsword.minstrel.cache.mutations.MutationQueue.enqueuePlaybackErrorReport]).
+ */
+interface PlaybackErrorsApi {
+    @POST("api/playback-errors")
+    suspend fun report(@Body body: PlaybackErrorReportRequest)
+}
+
+/**
+ * POST body for `/api/playback-errors`. `kind` is one of
+ * "zero_duration" / "load_failed" / "stalled"; server validates against
+ * the CHECK constraint enum. `detail` is optional free-text (Media3
+ * error message, etc.). `clientId` reuses the existing playback
+ * client identifier the device already sends with /api/plays/* so
+ * support can correlate reports across surfaces.
+ */
+@kotlinx.serialization.Serializable
+data class PlaybackErrorReportRequest(
+    @kotlinx.serialization.SerialName("track_id") val trackId: String,
+    val kind: String,
+    val detail: String? = null,
+    @kotlinx.serialization.SerialName("client_id") val clientId: String,
+)

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

@@ -21,6 +21,7 @@ object MutationKind {
     const val QUARANTINE_FLAG: String = "quarantine_flag"
     const val PLAY_OFFLINE: String = "play_offline"
     const val REQUEST_CANCEL: String = "request_cancel"
+    const val PLAYBACK_ERROR_REPORT: String = "playback_error_report"
 }
 
 /**
@@ -144,6 +145,13 @@ class MutationQueue @Inject constructor(
             ),
         ),
     )
+
+    suspend fun enqueuePlaybackErrorReport(payload: PlaybackErrorReportPayload): Long = dao.insert(
+        CachedMutationEntity(
+            kind = MutationKind.PLAYBACK_ERROR_REPORT,
+            payload = json.encodeToString(PlaybackErrorReportPayload.serializer(), payload),
+        ),
+    )
 }
 
 /**
@@ -203,3 +211,20 @@ data class PlayOfflinePayload(
  */
 @Serializable
 data class RequestCancelPayload(val requestId: String)
+
+/**
+ * Persisted payload for `MutationKind.PLAYBACK_ERROR_REPORT` — the
+ * `POST /api/playback-errors` call lost during a connectivity hiccup.
+ * The replayer re-fires the POST with this body. Server is naturally
+ * idempotent enough — multiple reports of the same (track, user,
+ * kind) become multiple rows in the admin inbox, which is acceptable
+ * (the admin can resolve them all with one action). `clientId` is
+ * the existing playback client identifier from AuthStore.
+ */
+@Serializable
+data class PlaybackErrorReportPayload(
+    val trackId: String,
+    val kind: String,
+    val detail: String? = null,
+    val clientId: String,
+)

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

@@ -5,6 +5,8 @@ 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.PlaybackErrorReportRequest
+import com.fabledsword.minstrel.api.endpoints.PlaybackErrorsApi
 import com.fabledsword.minstrel.api.endpoints.PlaylistsApi
 import com.fabledsword.minstrel.api.endpoints.QuarantineApi
 import com.fabledsword.minstrel.api.endpoints.RequestsApi
@@ -66,6 +68,7 @@ class MutationReplayer @Inject constructor(
     private val playlistsApi: PlaylistsApi = retrofit.create()
     private val eventsApi: EventsApi = retrofit.create()
     private val requestsApi: RequestsApi = retrofit.create()
+    private val playbackErrorsApi: PlaybackErrorsApi = retrofit.create()
 
     private val mutex = Mutex()
 
@@ -116,6 +119,7 @@ class MutationReplayer @Inject constructor(
         MutationKind.QUARANTINE_FLAG -> dispatchQuarantineFlag(row.payload)
         MutationKind.PLAY_OFFLINE -> dispatchPlayOffline(row.payload)
         MutationKind.REQUEST_CANCEL -> dispatchRequestCancel(row.payload)
+        MutationKind.PLAYBACK_ERROR_REPORT -> dispatchPlaybackErrorReport(row.payload)
         else -> {
             // Unknown kind — drop the row by claiming success so a
             // stale schema entry can't wedge the queue forever.
@@ -258,4 +262,24 @@ class MutationReplayer @Inject constructor(
             false
         }
     }
+
+    private suspend fun dispatchPlaybackErrorReport(payload: String): Boolean {
+        val decoded = json.decodeFromString(PlaybackErrorReportPayload.serializer(), payload)
+        return try {
+            playbackErrorsApi.report(
+                PlaybackErrorReportRequest(
+                    trackId = decoded.trackId,
+                    kind = decoded.kind,
+                    detail = decoded.detail,
+                    clientId = decoded.clientId,
+                ),
+            )
+            true
+        } catch (
+            @Suppress("TooGenericExceptionCaught", "SwallowedException") e: Throwable,
+        ) {
+            // Intentional swallow: row stays queued; next drain pass retries.
+            false
+        }
+    }
 }

android/app/src/main/java/com/fabledsword/minstrel/player/PlaybackErrorReporter.kt

@@ -13,19 +13,23 @@ import javax.inject.Singleton
 private const val DEBOUNCE_MS = 2_000L
 
 /**
- * Surfaces ExoPlayer track-load failures (404 / decoder failure /
- * premature EOS / network drop) as user-visible snackbar messages.
+ * Surfaces playback failures (load errors + zero-duration tracks) as
+ * user-visible snackbar messages AND admin-inbox reports.
  *
- * Without this the player just silently skips the dead track — the
- * worst kind of bug, because the user can't tell "this file is
- * broken" from "the app is flaky".
+ * Without the snackbar the player just silently skips the dead track —
+ * the worst kind of bug, because the user can't tell "this file is
+ * broken" from "the app is flaky". Without the admin report the
+ * operator never finds out the track is bad and the next user hits
+ * the same wall.
  *
- * Pattern mirrors Flutter's `playback_error_reporter.dart`: collect
- * per-error events from [PlayerController.playbackErrorEvents],
- * debounce in a 2s window, and emit "Couldn't play 'X' — skipping"
- * for a single error or "Skipped N unplayable tracks" when a burst
- * lands inside the window. Stops the user from seeing a stack of N
- * toasts on a network blip that fails N tracks in a row.
+ * The snackbar text mirrors Flutter's `playback_error_reporter.dart`:
+ * collect [PlayerController.playbackErrorEvents], debounce in a 2s
+ * window, emit "Couldn't play 'X' — skipping" for a single error or
+ * "Skipped N unplayable tracks" when a burst lands inside the window.
+ *
+ * The server report fires per event (no debounce) via
+ * [PlaybackErrorRepository], which handles the offline-first
+ * MutationQueue fallback per the standing rule for server writes.
  *
  * ShellScaffold collects [messages] into its existing snackbar host
  * so the reporter doesn't need its own UI surface. Constructed at
@@ -35,6 +39,7 @@ private const val DEBOUNCE_MS = 2_000L
 @Singleton
 class PlaybackErrorReporter @Inject constructor(
     private val playerController: PlayerController,
+    private val repository: PlaybackErrorRepository,
     @ApplicationScope private val scope: CoroutineScope,
 ) {
     private val outChannel = Channel<String>(Channel.BUFFERED)
@@ -46,8 +51,11 @@ class PlaybackErrorReporter @Inject constructor(
         scope.launch {
             val buffer = mutableListOf<String>()
             var debounceJob: kotlinx.coroutines.Job? = null
-            playerController.playbackErrorEvents.collect { title ->
-                buffer.add(title)
+            playerController.playbackErrorEvents.collect { event ->
+                // Fire-and-forget the server report — repository handles
+                // success/queue branching so callers don't see throws.
+                scope.launch { repository.report(event) }
+                buffer.add(event.title)
                 debounceJob?.cancel()
                 debounceJob = scope.launch {
                     delay(DEBOUNCE_MS)

android/app/src/main/java/com/fabledsword/minstrel/player/PlaybackErrorRepository.kt

@@ -0,0 +1,78 @@
+package com.fabledsword.minstrel.player
+
+import com.fabledsword.minstrel.api.endpoints.PlaybackErrorReportRequest
+import com.fabledsword.minstrel.api.endpoints.PlaybackErrorsApi
+import com.fabledsword.minstrel.auth.AuthStore
+import com.fabledsword.minstrel.cache.mutations.MutationQueue
+import com.fabledsword.minstrel.cache.mutations.PlaybackErrorReportPayload
+import retrofit2.Retrofit
+import retrofit2.create
+import java.util.UUID
+import javax.inject.Inject
+import javax.inject.Singleton
+
+/**
+ * Writes a [PlaybackErrorEvent] to the server. Follows the standing
+ * offline-first rule for server writes: try the POST first, fall back
+ * to the MutationQueue on transport failure so a tracked report is
+ * never lost.
+ *
+ * Reused client_id comes from [AuthStore.clientId] — the same UUID-
+ * per-install identifier the play-events reporter already sends, so
+ * support can correlate playback errors with the surrounding plays.
+ */
+@Singleton
+class PlaybackErrorRepository @Inject constructor(
+    private val authStore: AuthStore,
+    private val mutationQueue: MutationQueue,
+    retrofit: Retrofit,
+) {
+    private val api: PlaybackErrorsApi = retrofit.create()
+
+    /**
+     * Reports a playback failure. Returns [Outcome.ACCEPTED] on a 2xx,
+     * [Outcome.QUEUED] when the call was buffered to the MutationQueue
+     * for later replay. Never throws — callers don't need a try block.
+     */
+    suspend fun report(event: PlaybackErrorEvent): Outcome {
+        val clientId = resolveClientId()
+        val payload = PlaybackErrorReportPayload(
+            trackId = event.trackId,
+            kind = event.kind,
+            detail = event.detail,
+            clientId = clientId,
+        )
+        return try {
+            api.report(
+                PlaybackErrorReportRequest(
+                    trackId = payload.trackId,
+                    kind = payload.kind,
+                    detail = payload.detail,
+                    clientId = payload.clientId,
+                ),
+            )
+            Outcome.ACCEPTED
+        } catch (
+            @Suppress("TooGenericExceptionCaught", "SwallowedException") e: Throwable,
+        ) {
+            // Intentional swallow — see LikesRepository.toggleLike for
+            // the same offline-first rationale.
+            mutationQueue.enqueuePlaybackErrorReport(payload)
+            Outcome.QUEUED
+        }
+    }
+
+    /**
+     * Returns the persisted client id, generating + persisting one on
+     * first read. Mirrors [PlayEventsReporter.resolveClientId] — the
+     * value is shared across all client-id-carrying surfaces.
+     */
+    private fun resolveClientId(): String {
+        authStore.clientId.value?.let { return it }
+        val fresh = UUID.randomUUID().toString()
+        authStore.setClientId(fresh)
+        return fresh
+    }
+
+    enum class Outcome { ACCEPTED, QUEUED }
+}

android/app/src/main/java/com/fabledsword/minstrel/player/PlayerController.kt

@@ -68,13 +68,23 @@ class PlayerController @Inject constructor(
 
     /**
      * Per-error events for [com.fabledsword.minstrel.player.PlaybackErrorReporter].
-     * Emits the failing track's title (or `"Track"` when unknown) each
-     * time Media3's `onPlayerError` fires — not bound to the lifetime of
-     * any UI subscriber so a torn-down screen doesn't drop events.
-     * Conflated channel so backpressure can't stall the player loop.
+     * Fires twice: once when Media3's `onPlayerError` surfaces a load
+     * failure (decoder, transport, EOS), and once when the player
+     * reaches STATE_READY with a duration of zero / TIME_UNSET — the
+     * "track loaded but has no audio" case the user sees as the
+     * player sitting frozen on a track. Conflated channel so back-
+     * pressure can't stall the player loop.
      */
-    private val playbackErrorEventsChannel = Channel<String>(Channel.CONFLATED)
-    val playbackErrorEvents: Flow<String> = playbackErrorEventsChannel.receiveAsFlow()
+    private val playbackErrorEventsChannel = Channel<PlaybackErrorEvent>(Channel.CONFLATED)
+    val playbackErrorEvents: Flow<PlaybackErrorEvent> = playbackErrorEventsChannel.receiveAsFlow()
+
+    /**
+     * Tracks which queue index has already been evaluated for the
+     * zero-duration / load-failed checks so STATE_READY firing
+     * repeatedly (after every seek, pause/resume) doesn't re-emit
+     * the same error event. Reset on each onMediaItemTransition.
+     */
+    private var lastEvaluatedItemIndex: Int = -1
 
     /**
      * Stable queue snapshot kept in sync with the player's MediaItems —
@@ -234,12 +244,53 @@ class PlayerController @Inject constructor(
         controller.addListener(
             object : Player.Listener {
                 override fun onPlayerError(error: androidx.media3.common.PlaybackException) {
-                    val title = queueRefs
-                        .getOrNull(controller.currentMediaItemIndex)
-                        ?.title
-                        ?.takeIf { it.isNotEmpty() }
-                        ?: "Track"
-                    playbackErrorEventsChannel.trySend(title)
+                    val current = queueRefs.getOrNull(controller.currentMediaItemIndex)
+                    val title = current?.title?.takeIf { it.isNotEmpty() } ?: "Track"
+                    val trackId = current?.id ?: return
+                    playbackErrorEventsChannel.trySend(
+                        PlaybackErrorEvent(
+                            trackId = trackId,
+                            kind = "load_failed",
+                            title = title,
+                            detail = error.message,
+                        ),
+                    )
+                }
+
+                override fun onMediaItemTransition(
+                    mediaItem: androidx.media3.common.MediaItem?,
+                    reason: Int,
+                ) {
+                    // Reset the per-item evaluation guard so the new
+                    // item's STATE_READY transition gets a fresh check.
+                    lastEvaluatedItemIndex = -1
+                }
+
+                override fun onPlaybackStateChanged(playbackState: Int) {
+                    if (playbackState != Player.STATE_READY) return
+                    val idx = controller.currentMediaItemIndex
+                    if (idx == lastEvaluatedItemIndex) return
+                    lastEvaluatedItemIndex = idx
+                    val current = queueRefs.getOrNull(idx) ?: return
+                    val duration = controller.duration
+                    if (duration > 0L && duration != androidx.media3.common.C.TIME_UNSET) return
+                    // Zero-duration / unset-duration track: file decoded
+                    // to nothing playable. Fail fast — fire an error
+                    // event for the reporter (snackbar + server log) and
+                    // advance so the user doesn't sit on the dead track.
+                    playbackErrorEventsChannel.trySend(
+                        PlaybackErrorEvent(
+                            trackId = current.id,
+                            kind = "zero_duration",
+                            title = current.title.ifEmpty { "Track" },
+                            detail = "duration=$duration",
+                        ),
+                    )
+                    if (controller.hasNextMediaItem()) {
+                        controller.seekToNextMediaItem()
+                    } else {
+                        controller.stop()
+                    }
                 }
 
                 override fun onEvents(player: Player, events: Player.Events) {
@@ -368,3 +419,19 @@ class PlayerController @Inject constructor(
 // surfaces are driven by Media3's MediaSession callbacks separately;
 // they don't need this poll.
 private const val POSITION_POLL_INTERVAL_MS = 500L
+
+/**
+ * Structured playback failure event for [PlaybackErrorReporter]. Drives
+ * both the user-facing snackbar ("Couldn't play X — skipping") and the
+ * /api/playback-errors admin inbox.
+ *
+ * `kind` is one of "zero_duration" / "load_failed" / "stalled" matching
+ * the server's CHECK constraint. `detail` is free-text (the Media3
+ * error message, the observed duration value, etc.).
+ */
+data class PlaybackErrorEvent(
+    val trackId: String,
+    val kind: String,
+    val title: String,
+    val detail: String? = null,
+)