feat(M003/S01): play history tracking — schema, migration, recording hook

Migration 10:
- play_history table (audio_file_id FK, played_at DATETIME, CASCADE delete)
- play_count + last_played columns on audio_files (denormalized)
- Recreated track_metadata VIEW with play_count and last_played columns

Play recording:
- queue.recordPlay() inserts play_history row + updates denormalized columns
- Called from OnPlaybackFinished after queue advance completes
- Mutex released before DB write to avoid MaxOpenConns(1) deadlock
- Natural finish only — skip/stop does not count

Tests:
- TestMigration10PlayHistory: schema, columns, VIEW, round-trip verification
- All 49 smart playlist + 15 service + existing DB tests still pass

backend/queue/handlers.go

@@ -1,21 +1,28 @@
 package queue
 
 // OnPlaybackFinished is called when a track finishes playing naturally.
-// This drives the auto-advance behavior.
+// This drives the auto-advance behavior and records the play.
 func (q *Queue) OnPlaybackFinished() {
 	q.mu.Lock()
-	defer q.mu.Unlock()
 
 	if len(q.tracks) == 0 {
+		q.mu.Unlock()
+
 		return
 	}
 
+	// Capture the track that just finished before advancing.
+	finishedID := q.tracks[q.currentIndex].AudioFileID
+
 	// Repeat One: replay the current track.
 	if q.repeatMode == RepeatOne {
 		if q.playCurrentTrack() {
 			q.emitIndexChanged()
 		}
 
+		q.mu.Unlock()
+		q.recordPlay(finishedID)
+
 		return
 	}
 
@@ -23,6 +30,8 @@ func (q *Queue) OnPlaybackFinished() {
 	if nextIdx == -1 {
 		// Queue exhausted — this is the extension point for a future fallback playlist.
 		q.onQueueExhausted()
+		q.mu.Unlock()
+		q.recordPlay(finishedID)
 
 		return
 	}
@@ -32,9 +41,13 @@ func (q *Queue) OnPlaybackFinished() {
 
 	if !q.playCurrentTrack() {
 		q.currentIndex = prevIndex
+		q.mu.Unlock()
+		q.recordPlay(finishedID)
 
 		return
 	}
 
 	q.emitIndexChanged()
+	q.mu.Unlock()
+	q.recordPlay(finishedID)
 }