feat(db): add M2 likes schema (general_likes, _albums, _artists)

Three tables keyed on (user_id, entity_id) with liked_at. Per-table
indexes on (user_id, liked_at DESC) for the recently-liked feed.
sqlc queries cover like/unlike/list-rows/count/list-ids per entity.

internal/db/dbq/models.go

@@ -29,6 +29,24 @@ type Artist struct {
 	UpdatedAt pgtype.Timestamptz
 }
 
+type GeneralLike struct {
+	UserID  pgtype.UUID
+	TrackID pgtype.UUID
+	LikedAt pgtype.Timestamptz
+}
+
+type GeneralLikesAlbum struct {
+	UserID  pgtype.UUID
+	AlbumID pgtype.UUID
+	LikedAt pgtype.Timestamptz
+}
+
+type GeneralLikesArtist struct {
+	UserID   pgtype.UUID
+	ArtistID pgtype.UUID
+	LikedAt  pgtype.Timestamptz
+}
+
 type PlayEvent struct {
 	ID                  pgtype.UUID
 	UserID              pgtype.UUID