feat: initial implementation of metadata aggregator

- gRPC service with MusicBrainz provider
- PostgreSQL schema with migrations
- Service layer with database-first caching
- Repository pattern for data access
- YAML configuration support
- Research documentation for 17 music metadata projects

database/migrations/002_initial_schema.down.sql

@@ -0,0 +1,33 @@
+DROP INDEX IF EXISTS idx_playlist_tracks_position;
+DROP INDEX IF EXISTS idx_lyrics_track_id;
+DROP INDEX IF EXISTS idx_genres_name;
+DROP INDEX IF EXISTS idx_albums_release_date;
+DROP INDEX IF EXISTS idx_albums_source;
+DROP INDEX IF EXISTS idx_albums_upc;
+DROP INDEX IF EXISTS idx_tracks_source;
+DROP INDEX IF EXISTS idx_tracks_isrc;
+DROP INDEX IF EXISTS idx_artists_source;
+DROP INDEX IF EXISTS idx_artists_name;
+
+DROP TABLE IF EXISTS track_external_ids;
+DROP TABLE IF EXISTS album_external_ids;
+DROP TABLE IF EXISTS artist_external_ids;
+
+DROP TABLE IF EXISTS playlist_tracks;
+DROP TABLE IF EXISTS playlists;
+DROP TABLE IF EXISTS lyrics;
+
+DROP TABLE IF EXISTS similar_artists;
+DROP TABLE IF EXISTS album_genres;
+DROP TABLE IF EXISTS artist_genres;
+DROP TABLE IF EXISTS work_artists;
+DROP TABLE IF EXISTS album_tracks;
+DROP TABLE IF EXISTS album_artists;
+DROP TABLE IF EXISTS track_artists;
+
+DROP TABLE IF EXISTS genres;
+DROP TABLE IF EXISTS albums;
+DROP TABLE IF EXISTS labels;
+DROP TABLE IF EXISTS tracks;
+DROP TABLE IF EXISTS works;
+DROP TABLE IF EXISTS artists;