feat(android): port cached_mutations (M8 4.2 slice 6)

Mirrors flutter_client/lib/cache/db.dart's CachedMutations — the
offline-write queue that MutationQueue.enqueue() inserts into when a
server-write fails with IOException and MutationReplayer.drain() pops
from when connectivity returns (Phase 12.2).

`kind` is a string registered in `MutationKind` (Phase 12.2) so the
replayer can map to the right handler. `payload` is JSON-serialized
args. Unknown kinds get dropped at drain time rather than wedging.

DAO surface tailored to the replayer:
  - observePendingCount: Flow<Int> for the offline-indicator badge
  - getAll: FIFO list for drain (id ASC = oldest first)
  - insert: returns the autoGenerate'd id
  - recordAttempt(id, instant): atomic increment + lastAttemptAt set
  - delete(id) / clear

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

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

@@ -7,6 +7,7 @@ import com.fabledsword.minstrel.cache.db.dao.AudioCacheIndexDao
 import com.fabledsword.minstrel.cache.db.dao.CachedAlbumDao
 import com.fabledsword.minstrel.cache.db.dao.CachedArtistDao
 import com.fabledsword.minstrel.cache.db.dao.CachedLikeDao
+import com.fabledsword.minstrel.cache.db.dao.CachedMutationDao
 import com.fabledsword.minstrel.cache.db.dao.CachedPlaylistDao
 import com.fabledsword.minstrel.cache.db.dao.CachedPlaylistTrackDao
 import com.fabledsword.minstrel.cache.db.dao.CachedQuarantineDao
@@ -16,6 +17,7 @@ import com.fabledsword.minstrel.cache.db.entities.AudioCacheIndexEntity
 import com.fabledsword.minstrel.cache.db.entities.CachedAlbumEntity
 import com.fabledsword.minstrel.cache.db.entities.CachedArtistEntity
 import com.fabledsword.minstrel.cache.db.entities.CachedLikeEntity
+import com.fabledsword.minstrel.cache.db.entities.CachedMutationEntity
 import com.fabledsword.minstrel.cache.db.entities.CachedPlaylistEntity
 import com.fabledsword.minstrel.cache.db.entities.CachedPlaylistTrackEntity
 import com.fabledsword.minstrel.cache.db.entities.CachedQuarantineEntity
@@ -46,6 +48,7 @@ import com.fabledsword.minstrel.cache.db.entities.SyncMetadataEntity
         CachedPlaylistTrackEntity::class,
         CachedQuarantineEntity::class,
         AudioCacheIndexEntity::class,
+        CachedMutationEntity::class,
     ],
     version = 1,
     exportSchema = true,
@@ -61,4 +64,5 @@ abstract class AppDatabase : RoomDatabase() {
     abstract fun cachedPlaylistTrackDao(): CachedPlaylistTrackDao
     abstract fun cachedQuarantineDao(): CachedQuarantineDao
     abstract fun audioCacheIndexDao(): AudioCacheIndexDao
+    abstract fun cachedMutationDao(): CachedMutationDao
 }

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

@@ -0,0 +1,34 @@
+package com.fabledsword.minstrel.cache.db.dao
+
+import androidx.room.Dao
+import androidx.room.Insert
+import androidx.room.Query
+import com.fabledsword.minstrel.cache.db.entities.CachedMutationEntity
+import kotlinx.coroutines.flow.Flow
+import kotlinx.datetime.Instant
+
+@Dao
+interface CachedMutationDao {
+    /** Pending count for the offline-indicator badge. */
+    @Query("SELECT COUNT(*) FROM cached_mutations")
+    fun observePendingCount(): Flow<Int>
+
+    /** FIFO drain order so the replayer processes oldest first. */
+    @Query("SELECT * FROM cached_mutations ORDER BY id ASC")
+    suspend fun getAll(): List<CachedMutationEntity>
+
+    @Insert
+    suspend fun insert(row: CachedMutationEntity): Long
+
+    @Query(
+        "UPDATE cached_mutations SET attempts = attempts + 1, lastAttemptAt = :at " +
+            "WHERE id = :id",
+    )
+    suspend fun recordAttempt(id: Long, at: Instant)
+
+    @Query("DELETE FROM cached_mutations WHERE id = :id")
+    suspend fun delete(id: Long)
+
+    @Query("DELETE FROM cached_mutations")
+    suspend fun clear()
+}

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

@@ -0,0 +1,32 @@
+package com.fabledsword.minstrel.cache.db.entities
+
+import androidx.room.Entity
+import androidx.room.PrimaryKey
+import kotlinx.datetime.Clock
+import kotlinx.datetime.Instant
+
+/**
+ * One row per pending offline-write. Mirrors
+ * `flutter_client/lib/cache/db.dart`'s `CachedMutations` Drift table.
+ *
+ * MutationQueue.enqueue() inserts a row when a server-write fails with
+ * an IOException; MutationReplayer.drain() pops and re-attempts each
+ * row when connectivity returns (Phase 12.2).
+ *
+ * `kind` is a stable string registered in `MutationKind` — the replayer
+ * looks up the handler in a kind→suspend-function map. Unknown kinds
+ * are dropped so a malformed row can't wedge the queue forever.
+ *
+ * `payload` is the JSON-serialized args needed to replay the REST call.
+ * Shape is kind-specific; the kind enum doubles as a tag for which
+ * decoder to apply at drain time.
+ */
+@Entity(tableName = "cached_mutations")
+data class CachedMutationEntity(
+    @PrimaryKey(autoGenerate = true) val id: Long = 0,
+    val kind: String,
+    val payload: String,
+    val createdAt: Instant = Clock.System.now(),
+    val lastAttemptAt: Instant? = null,
+    val attempts: Int = 0,
+)