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

internal/provider/musicbrainz/errors.go

@@ -0,0 +1,8 @@
+package musicbrainz
+
+import "errors"
+
+var (
+	ErrNotFound    = errors.New("not found")
+	ErrRateLimited = errors.New("rate limited")
+)