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/audio_files.sql

@@ -11,6 +11,8 @@ CREATE TABLE IF NOT EXISTS audio_files (
   file_size int NOT NULL DEFAULT 0,
   basename text NOT NULL DEFAULT '',
   library_id int NOT NULL DEFAULT 0,
+  play_count int NOT NULL DEFAULT 0,
+  last_played datetime,
   FOREIGN KEY(file_type_id) REFERENCES file_types(id),
   FOREIGN KEY(recording_id) REFERENCES recordings(id),
   FOREIGN KEY(library_id) REFERENCES libraries(id)