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/repository/errors.go

@@ -0,0 +1,5 @@
+package repository
+
+import "errors"
+
+var ErrNotFound = errors.New("not found")