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/domain/types.go

@@ -0,0 +1,84 @@
+package domain
+
+import "time"
+
+type Artist struct {
+	ID            string
+	Name          string
+	SortName      string
+	Type          string
+	Country       string
+	FormedDate    *time.Time
+	DisbandedDate *time.Time
+	Description   string
+	ImageURL      string
+	Genres        []Genre
+	ExternalIDs   []ExternalID
+}
+
+type Album struct {
+	ID          string
+	Title       string
+	Type        string
+	ReleaseDate *time.Time
+	UPC         string
+	TotalTracks int
+	TotalDiscs  int
+	CoverURL    string
+	Artists     []ArtistCredit
+	Label       *Label
+	Genres      []Genre
+	ExternalIDs []ExternalID
+}
+
+type Track struct {
+	ID          string
+	Title       string
+	DurationMs  int
+	ISRC        string
+	Explicit    bool
+	DiscNumber  int
+	TrackNumber int
+	Artists     []ArtistCredit
+	Work        *Work
+	ExternalIDs []ExternalID
+}
+
+type Work struct {
+	ID        string
+	Title     string
+	Type      string
+	Language  string
+	Composers []ArtistCredit
+}
+
+type Label struct {
+	ID      string
+	Name    string
+	Country string
+}
+
+type Genre struct {
+	ID   string
+	Name string
+}
+
+type ArtistCredit struct {
+	Artist     Artist
+	Role       string
+	Position   int
+	JoinPhrase string
+}
+
+type ExternalID struct {
+	Source   string
+	SourceID string
+	URL      string
+}
+
+type SearchResult[T any] struct {
+	Items  []T
+	Total  int
+	Limit  int
+	Offset int
+}