feat(android): port audio_cache_index (M8 4.2 slice 5)

Mirrors flutter_client/lib/cache/db.dart's AudioCacheIndex — one row
per fully-downloaded audio file. Drives the 2-bucket LRU eviction
policy that Phase 12's AudioCacheEvictionWorker will execute.

DAO surface tailored to the eviction worker:
  - totalBytes / bytesBySource — sum-of-sizeBytes for cap checks
  - evictionCandidates(source) — oldest lastPlayedAt within a source
    bucket, NULL lastPlayedAt sorted first (never-played candidates
    evict before played ones)
  - touchLastPlayed(trackId, instant) — single-column update from
    the player on every "ready+playing" transition
  - bulk delete by track-id list for batch evictions

CacheSource enum (MANUAL / INCIDENTAL / AUTO_PREFETCH) ported in
Task 4.1's TypeConverters.

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

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

@@ -3,6 +3,7 @@ package com.fabledsword.minstrel.cache.db
 import androidx.room.Database
 import androidx.room.RoomDatabase
 import androidx.room.TypeConverters
+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
@@ -11,6 +12,7 @@ import com.fabledsword.minstrel.cache.db.dao.CachedPlaylistTrackDao
 import com.fabledsword.minstrel.cache.db.dao.CachedQuarantineDao
 import com.fabledsword.minstrel.cache.db.dao.CachedTrackDao
 import com.fabledsword.minstrel.cache.db.dao.SyncMetadataDao
+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
@@ -43,6 +45,7 @@ import com.fabledsword.minstrel.cache.db.entities.SyncMetadataEntity
         CachedPlaylistEntity::class,
         CachedPlaylistTrackEntity::class,
         CachedQuarantineEntity::class,
+        AudioCacheIndexEntity::class,
     ],
     version = 1,
     exportSchema = true,
@@ -57,4 +60,5 @@ abstract class AppDatabase : RoomDatabase() {
     abstract fun cachedPlaylistDao(): CachedPlaylistDao
     abstract fun cachedPlaylistTrackDao(): CachedPlaylistTrackDao
     abstract fun cachedQuarantineDao(): CachedQuarantineDao
+    abstract fun audioCacheIndexDao(): AudioCacheIndexDao
 }

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

@@ -0,0 +1,60 @@
+package com.fabledsword.minstrel.cache.db.dao
+
+import androidx.room.Dao
+import androidx.room.Insert
+import androidx.room.OnConflictStrategy
+import androidx.room.Query
+import com.fabledsword.minstrel.cache.db.CacheSource
+import com.fabledsword.minstrel.cache.db.entities.AudioCacheIndexEntity
+import kotlinx.coroutines.flow.Flow
+
+@Dao
+interface AudioCacheIndexDao {
+    /** Used to render the "Downloaded" / "Offline" listing. */
+    @Query("SELECT * FROM audio_cache_index ORDER BY lastPlayedAt DESC NULLS LAST")
+    fun observeAll(): Flow<List<AudioCacheIndexEntity>>
+
+    @Query("SELECT * FROM audio_cache_index WHERE trackId = :trackId")
+    suspend fun get(trackId: String): AudioCacheIndexEntity?
+
+    @Query("SELECT * FROM audio_cache_index WHERE trackId IN (:trackIds)")
+    suspend fun getByTrackIds(trackIds: List<String>): List<AudioCacheIndexEntity>
+
+    /** All cached track IDs — bucket-membership lookup for eviction. */
+    @Query("SELECT trackId FROM audio_cache_index")
+    suspend fun allCachedTrackIds(): List<String>
+
+    /** Eviction-candidate ordering — oldest lastPlayedAt first within a source. */
+    @Query(
+        "SELECT * FROM audio_cache_index " +
+            "WHERE source = :source " +
+            "ORDER BY lastPlayedAt ASC NULLS FIRST, cachedAt ASC",
+    )
+    suspend fun evictionCandidates(source: CacheSource): List<AudioCacheIndexEntity>
+
+    /** Sum of bytes across rows; the rolling/liked bucket caps compare against this. */
+    @Query("SELECT COALESCE(SUM(sizeBytes), 0) FROM audio_cache_index")
+    suspend fun totalBytes(): Long
+
+    @Query(
+        "SELECT COALESCE(SUM(sizeBytes), 0) FROM audio_cache_index WHERE source = :source",
+    )
+    suspend fun bytesBySource(source: CacheSource): Long
+
+    @Insert(onConflict = OnConflictStrategy.REPLACE)
+    suspend fun upsert(row: AudioCacheIndexEntity)
+
+    @Query(
+        "UPDATE audio_cache_index SET lastPlayedAt = :at WHERE trackId = :trackId",
+    )
+    suspend fun touchLastPlayed(trackId: String, at: kotlinx.datetime.Instant)
+
+    @Query("DELETE FROM audio_cache_index WHERE trackId = :trackId")
+    suspend fun deleteByTrackId(trackId: String)
+
+    @Query("DELETE FROM audio_cache_index WHERE trackId IN (:trackIds)")
+    suspend fun deleteByTrackIds(trackIds: List<String>)
+
+    @Query("DELETE FROM audio_cache_index")
+    suspend fun clear()
+}

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

@@ -0,0 +1,31 @@
+package com.fabledsword.minstrel.cache.db.entities
+
+import androidx.room.Entity
+import androidx.room.PrimaryKey
+import com.fabledsword.minstrel.cache.db.CacheSource
+import kotlinx.datetime.Clock
+import kotlinx.datetime.Instant
+
+/**
+ * One row per fully-downloaded audio file. Mirrors
+ * `flutter_client/lib/cache/db.dart`'s `AudioCacheIndex` Drift table.
+ *
+ * Drives the 2-bucket LRU eviction (Phase 12 AudioCacheEvictionWorker):
+ *   - `incidental` files (streamed-and-cached side effect) evict first
+ *   - `autoPrefetch` second
+ *   - `manual` (user explicitly pinned) evict last
+ *
+ * `cachedAt` = when we downloaded. `lastPlayedAt` = when the user last
+ * played it; drives the offline "recently played" ordering and is the
+ * tiebreaker for LRU eviction within a source bucket. Nullable on
+ * brand-new rows until the first post-cache play touches them.
+ */
+@Entity(tableName = "audio_cache_index")
+data class AudioCacheIndexEntity(
+    @PrimaryKey val trackId: String,
+    val path: String,
+    val sizeBytes: Int,
+    val cachedAt: Instant = Clock.System.now(),
+    val lastPlayedAt: Instant? = null,
+    val source: CacheSource,
+)