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/database/sql/schemas/track_metadata_view.sql

@@ -23,7 +23,9 @@ SELECT
     af.channels,
     af.bitrate,
     af.file_size,
-    af.library_id
+    af.library_id,
+    af.play_count,
+    af.last_played
 FROM audio_files af
 LEFT JOIN recordings r ON af.recording_id = r.id
 LEFT JOIN artist_credit ac ON r.artist_credit_id = ac.id