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/provider.go

@@ -0,0 +1,25 @@
+package provider
+
+import (
+	"context"
+
+	"github.com/metadata-agregator/internal/domain"
+)
+
+type Provider interface {
+	Name() string
+
+	GetArtist(ctx context.Context, id string) (*domain.Artist, error)
+	SearchArtists(ctx context.Context, query string, limit, offset int) (*domain.SearchResult[domain.Artist], error)
+
+	GetAlbum(ctx context.Context, id string) (*domain.Album, error)
+	GetArtistAlbums(ctx context.Context, artistID string, limit, offset int) (*domain.SearchResult[domain.Album], error)
+
+	GetTrack(ctx context.Context, id string) (*domain.Track, error)
+	GetAlbumTracks(ctx context.Context, albumID string) ([]domain.Track, error)
+	GetTrackByISRC(ctx context.Context, isrc string) (*domain.Track, error)
+
+	GetLabel(ctx context.Context, id string) (*domain.Label, error)
+
+	GetWork(ctx context.Context, id string) (*domain.Work, error)
+}