feat(api): extend ArtistRef/AlbumRef + add HomePayload for M6a

- ArtistRef gains SortName and CoverURL fields
- AlbumRef gains SortTitle field
- albumRefFrom now populates SortTitle from dbq.Album.SortTitle
- artistRefFrom now populates SortName from dbq.Artist.SortName
- New artistRefFromCovered helper builds CoverURL from nullable cover_album_id
- New HomePayload view type for GET /api/home response body

internal/api/convert.go

@@ -75,16 +75,31 @@ func streamURL(trackID pgtype.UUID) string {
 	return "/api/tracks/" + uuidToString(trackID) + "/stream"
 }
 
-// artistRefFrom projects a dbq.Artist into an ArtistRef. albumCount must be
-// pre-computed by the caller (one query per artist in lists).
+// artistRefFrom projects a dbq.Artist into an ArtistRef without cover.
+// albumCount must be pre-computed by the caller. Used by code paths that
+// don't have a representative-album lookup at hand (artist detail, search,
+// liked-artists list).
 func artistRefFrom(a dbq.Artist, albumCount int) ArtistRef {
 	return ArtistRef{
 		ID:         uuidToString(a.ID),
 		Name:       a.Name,
+		SortName:   a.SortName,
 		AlbumCount: albumCount,
 	}
 }
 
+// artistRefFromCovered projects a dbq.Artist into an ArtistRef with the
+// derived CoverURL populated from a representative album id (nullable).
+// Used by /api/home and /api/artists?sort=alpha which carry the lookup
+// in the same query.
+func artistRefFromCovered(a dbq.Artist, albumCount int, coverAlbumID pgtype.UUID) ArtistRef {
+	ref := artistRefFrom(a, albumCount)
+	if coverAlbumID.Valid {
+		ref.CoverURL = coverURL(coverAlbumID)
+	}
+	return ref
+}
+
 // albumRefFrom projects a dbq.Album into an AlbumRef. artistName must be
 // pre-resolved because Album only carries artist_id. trackCount and
 // durationSec may be 0 when the caller does not compute them.
@@ -92,6 +107,7 @@ func albumRefFrom(a dbq.Album, artistName string, trackCount, durationSec int) A
 	return AlbumRef{
 		ID:          uuidToString(a.ID),
 		Title:       a.Title,
+		SortTitle:   a.SortTitle,
 		ArtistID:    uuidToString(a.ArtistID),
 		ArtistName:  artistName,
 		Year:        yearFromDate(a.ReleaseDate),