feat: sync OpenSubsonic replay gain

Store the per-track replay gain a server reports on OpenSubsonic's
`Child.replayGain` so playback can normalise loudness later.

- add trackGain/albumGain/trackPeak/albumPeak (REAL) to the songs table
  and the 27 -> 28 migration
- parse the nested `<replayGain>` element, folding the child-element
  dispatch into a single `songChild` shared by the search and album paths
- teach the sync upsert to bind Float, which it previously rejected

baseGain and fallbackGain are omitted: the servers that send the object
do not populate them, so they would only ever be null columns.

Nothing applies the values yet; this slice only mirrors them.

app/src/main/kotlin/com/subtracks/data/db/Migrations.kt

@@ -384,6 +384,16 @@ val MIGRATION_26_27 =
         }
     }
 
+val MIGRATION_27_28 =
+    object : Migration(27, 28) {
+        override suspend fun migrate(connection: SQLiteConnection) {
+            connection.execSQL("ALTER TABLE `songs` ADD COLUMN `trackGain` REAL")
+            connection.execSQL("ALTER TABLE `songs` ADD COLUMN `albumGain` REAL")
+            connection.execSQL("ALTER TABLE `songs` ADD COLUMN `trackPeak` REAL")
+            connection.execSQL("ALTER TABLE `songs` ADD COLUMN `albumPeak` REAL")
+        }
+    }
+
 // Mirrors the FTS5 table and content-sync triggers Room generates for the @Fts5 entities.
 private suspend fun createSearchIndex(
     connection: SQLiteConnection,
@@ -444,4 +454,5 @@ val MIGRATIONS: Array<Migration> =
         MIGRATION_24_25,
         MIGRATION_25_26,
         MIGRATION_26_27,
+        MIGRATION_27_28,
     )

app/src/main/kotlin/com/subtracks/data/db/SubtracksDatabase.kt

@@ -49,7 +49,7 @@ import com.subtracks.data.model.UpNextEntry
         SongDownload::class,
         PendingAction::class,
     ],
-    version = 27,
+    version = 28,
     exportSchema = true,
 )
 @ColumnTypeConverters(

app/src/main/kotlin/com/subtracks/data/model/Models.kt

@@ -264,6 +264,10 @@ data class Song(
     @ColumnInfo(defaultValue = "0") val created: Long = 0,
     @ColumnInfo(defaultValue = "0") val playCount: Long = 0,
     val played: Long? = null,
+    val trackGain: Float? = null,
+    val albumGain: Float? = null,
+    val trackPeak: Float? = null,
+    val albumPeak: Float? = null,
     @Ignore val genres: List<String> = emptyList(),
 )
 

app/src/main/kotlin/com/subtracks/data/source/subsonic/SubsonicSource.kt

@@ -207,7 +207,7 @@ class SubsonicSource(
                         ),
                     tag = "song",
                     create = { SubsonicXml.songDraft(id, it) },
-                    onChild = { song, name, attrs -> if (name == "genres") song.addGenre(attrs) },
+                    onChild = SubsonicXml::songChild,
                 ).collect { batch ->
                     raw += batch.size
                     val accepted = batch.map { it.toSong() }.filter { it.id.isNotEmpty() }
@@ -267,7 +267,7 @@ class SubsonicSource(
                                         params = mapOf("id" to albumId),
                                         tag = "song",
                                         create = { SubsonicXml.songDraft(id, it) },
-                                        onChild = { song, name, attrs -> if (name == "genres") song.addGenre(attrs) },
+                                        onChild = SubsonicXml::songChild,
                                     ).collect { batch ->
                                         val accepted = batch.map { it.toSong() }.filter { it.id.isNotEmpty() }
                                         accept(accepted)

app/src/main/kotlin/com/subtracks/data/source/subsonic/SubsonicXml.kt

@@ -78,6 +78,17 @@ object SubsonicXml {
             played = attrs.dateAttr("played"),
         )
 
+    internal fun songChild(
+        entity: SongDraft,
+        name: String,
+        attrs: Attributes,
+    ) {
+        when (name) {
+            "genres" -> entity.addGenre(attrs)
+            "replayGain" -> entity.addReplayGain(attrs)
+        }
+    }
+
     internal fun playlist(
         sourceId: Long,
         attrs: Attributes,
@@ -184,6 +195,10 @@ internal class SongDraft(
     private val created: Long = 0L,
     private val playCount: Long = 0L,
     private val played: Long? = null,
+    private var trackGain: Float? = null,
+    private var albumGain: Float? = null,
+    private var trackPeak: Float? = null,
+    private var albumPeak: Float? = null,
     private val genres: MutableList<String> = mutableListOf(),
 ) {
     fun addGenre(attrs: Attributes) {
@@ -194,6 +209,13 @@ internal class SongDraft(
             ?.let { genres += it }
     }
 
+    fun addReplayGain(attrs: Attributes) {
+        trackGain = attrs.floatAttr("trackGain")
+        albumGain = attrs.floatAttr("albumGain")
+        trackPeak = attrs.floatAttr("trackPeak")
+        albumPeak = attrs.floatAttr("albumPeak")
+    }
+
     fun toSong(): Song =
         Song(
             sourceId = sourceId,
@@ -211,6 +233,10 @@ internal class SongDraft(
             created = created,
             playCount = playCount,
             played = played,
+            trackGain = trackGain,
+            albumGain = albumGain,
+            trackPeak = trackPeak,
+            albumPeak = albumPeak,
             genres = genres.toList(),
         )
 }
@@ -219,6 +245,8 @@ internal fun Attributes.attr(name: String): String = getValue(name) ?: ""
 
 internal fun Attributes.longAttr(name: String): Long? = attr(name).takeIf { it.isNotEmpty() }?.toLongOrNull()
 
+internal fun Attributes.floatAttr(name: String): Float? = attr(name).takeIf { it.isNotEmpty() }?.toFloatOrNull()
+
 internal fun Attributes.dateAttr(name: String): Long? = attr(name).takeIf { it.isNotEmpty() }?.let(IsoDate::parse)
 
 private const val ROOT_TAG = "subsonic-response"

app/src/main/kotlin/com/subtracks/data/sync/SyncService.kt

@@ -314,6 +314,10 @@ private val songColumns =
         "created",
         "playCount",
         "played",
+        "trackGain",
+        "albumGain",
+        "trackPeak",
+        "albumPeak",
     )
 
 private val playlistColumns = listOf("sourceId", "id", "name", "comment", "coverArt", "songCount", "created", "changed", "duration")
@@ -358,6 +362,10 @@ private fun Song.values() =
         created,
         playCount,
         played,
+        trackGain,
+        albumGain,
+        trackPeak,
+        albumPeak,
     )
 
 private fun SongGenre.values() = listOf<Any?>(sourceId, songId, position, genre)
@@ -401,6 +409,7 @@ private fun SQLiteStatement.bind(
         is Int -> bindLong(index, value.toLong())
         is Boolean -> bindLong(index, if (value) 1L else 0L)
         is Double -> bindDouble(index, value)
+        is Float -> bindDouble(index, value.toDouble())
         is String -> bindText(index, value)
         else -> error("Cannot bind ${value::class}")
     }