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/repository.go

@@ -0,0 +1,29 @@
+package repository
+
+import (
+	"context"
+
+	"github.com/metadata-agregator/internal/domain"
+)
+
+type ArtistRepository interface {
+	GetByID(ctx context.Context, id string) (*domain.Artist, error)
+	GetByExternalID(ctx context.Context, source, sourceID string) (*domain.Artist, error)
+	Search(ctx context.Context, query string, limit, offset int) (*domain.SearchResult[domain.Artist], error)
+	Save(ctx context.Context, artist *domain.Artist) error
+}
+
+type AlbumRepository interface {
+	GetByID(ctx context.Context, id string) (*domain.Album, error)
+	GetByExternalID(ctx context.Context, source, sourceID string) (*domain.Album, error)
+	GetByArtistID(ctx context.Context, artistID string, limit, offset int) (*domain.SearchResult[domain.Album], error)
+	Save(ctx context.Context, album *domain.Album) error
+}
+
+type TrackRepository interface {
+	GetByID(ctx context.Context, id string) (*domain.Track, error)
+	GetByExternalID(ctx context.Context, source, sourceID string) (*domain.Track, error)
+	GetByISRC(ctx context.Context, isrc string) (*domain.Track, error)
+	GetByAlbumID(ctx context.Context, albumID string) ([]domain.Track, error)
+	Save(ctx context.Context, track *domain.Track) error
+}