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/service/metadata.go

@@ -0,0 +1,142 @@
+package service
+
+import (
+	"context"
+	"errors"
+
+	"github.com/metadata-agregator/internal/domain"
+	"github.com/metadata-agregator/internal/provider"
+	"github.com/metadata-agregator/internal/repository"
+)
+
+type MetadataService struct {
+	artists  repository.ArtistRepository
+	albums   repository.AlbumRepository
+	tracks   repository.TrackRepository
+	provider provider.Provider
+}
+
+func NewMetadataService(
+	artists repository.ArtistRepository,
+	albums repository.AlbumRepository,
+	tracks repository.TrackRepository,
+	prov provider.Provider,
+) *MetadataService {
+	return &MetadataService{
+		artists:  artists,
+		albums:   albums,
+		tracks:   tracks,
+		provider: prov,
+	}
+}
+
+func (s *MetadataService) GetArtist(ctx context.Context, id string) (*domain.Artist, error) {
+	artist, err := s.artists.GetByExternalID(ctx, s.provider.Name(), id)
+	if err == nil {
+		return artist, nil
+	}
+	if !errors.Is(err, repository.ErrNotFound) {
+		return nil, err
+	}
+
+	artist, err = s.provider.GetArtist(ctx, id)
+	if err != nil {
+		return nil, err
+	}
+
+	if saveErr := s.artists.Save(ctx, artist); saveErr != nil {
+		return artist, nil
+	}
+
+	return artist, nil
+}
+
+func (s *MetadataService) SearchArtists(ctx context.Context, query string, limit, offset int) (*domain.SearchResult[domain.Artist], error) {
+	result, err := s.artists.Search(ctx, query, limit, offset)
+	if err == nil && len(result.Items) > 0 {
+		return result, nil
+	}
+
+	return s.provider.SearchArtists(ctx, query, limit, offset)
+}
+
+func (s *MetadataService) GetAlbum(ctx context.Context, id string) (*domain.Album, error) {
+	album, err := s.albums.GetByExternalID(ctx, s.provider.Name(), id)
+	if err == nil {
+		return album, nil
+	}
+	if !errors.Is(err, repository.ErrNotFound) {
+		return nil, err
+	}
+
+	album, err = s.provider.GetAlbum(ctx, id)
+	if err != nil {
+		return nil, err
+	}
+
+	if saveErr := s.albums.Save(ctx, album); saveErr != nil {
+		return album, nil
+	}
+
+	return album, nil
+}
+
+func (s *MetadataService) GetArtistAlbums(ctx context.Context, artistID string, limit, offset int) (*domain.SearchResult[domain.Album], error) {
+	result, err := s.albums.GetByArtistID(ctx, artistID, limit, offset)
+	if err == nil && len(result.Items) > 0 {
+		return result, nil
+	}
+
+	return s.provider.GetArtistAlbums(ctx, artistID, limit, offset)
+}
+
+func (s *MetadataService) GetTrack(ctx context.Context, id string) (*domain.Track, error) {
+	track, err := s.tracks.GetByExternalID(ctx, s.provider.Name(), id)
+	if err == nil {
+		return track, nil
+	}
+	if !errors.Is(err, repository.ErrNotFound) {
+		return nil, err
+	}
+
+	track, err = s.provider.GetTrack(ctx, id)
+	if err != nil {
+		return nil, err
+	}
+
+	if saveErr := s.tracks.Save(ctx, track); saveErr != nil {
+		return track, nil
+	}
+
+	return track, nil
+}
+
+func (s *MetadataService) GetTrackByISRC(ctx context.Context, isrc string) (*domain.Track, error) {
+	track, err := s.tracks.GetByISRC(ctx, isrc)
+	if err == nil {
+		return track, nil
+	}
+	if !errors.Is(err, repository.ErrNotFound) {
+		return nil, err
+	}
+
+	track, err = s.provider.GetTrackByISRC(ctx, isrc)
+	if err != nil {
+		return nil, err
+	}
+
+	if saveErr := s.tracks.Save(ctx, track); saveErr != nil {
+		return track, nil
+	}
+
+	return track, nil
+}
+
+func (s *MetadataService) GetAlbumTracks(ctx context.Context, albumID string) ([]domain.Track, error) {
+	tracks, err := s.tracks.GetByAlbumID(ctx, albumID)
+	if err == nil && len(tracks) > 0 {
+		return tracks, nil
+	}
+
+	return s.provider.GetAlbumTracks(ctx, albumID)
+}