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/play_history.sql

@@ -0,0 +1,9 @@
+CREATE TABLE IF NOT EXISTS play_history (
+    id INTEGER PRIMARY KEY,
+    audio_file_id INTEGER NOT NULL,
+    played_at DATETIME NOT NULL DEFAULT (datetime('now')),
+    FOREIGN KEY(audio_file_id) REFERENCES audio_files(id) ON DELETE CASCADE
+);
+
+CREATE INDEX IF NOT EXISTS idx_play_history_audio_file_id
+    ON play_history(audio_file_id);