refactor(server): unify stream URL builders + MIME tables + cover-path helper

Closes Scribe #614, #615, server half of #616 surfaced by the 2026-06-04 divergent-provider audit.

- streamURL helper now used everywhere /api/tracks/{id}/stream is built (was inline concat in playlists.go and cast_token.go); add streamURLWithExt for the .ext cast variant.

- audioContentType in media.go is the canonical file_format -> MIME lookup; mimeForFormat in cast_token.go is now a thin wrapper that overrides the unknown-format fallback to audio/mpeg (Sonos rejects octet-stream). Adds mpeg/vorbis/wave aliases. Subsonic's contentTypeForFormat stays frozen per docs.

- coverart.ResolveAlbumPath extracted; api and subsonic both delegate to it.

internal/subsonic/stream.go

@@ -130,21 +130,10 @@ func (m *mediaHandlers) handleGetCoverArt(w http.ResponseWriter, r *http.Request
 	WriteFail(w, r, ErrDataNotFound, "Cover art not found")
 }
 
-// resolveAlbumCoverPath returns the filesystem path to the album's cover art,
-// preferring an explicit cover_art_path (set by the scanner in a future
-// milestone) and falling back to a sidecar image next to any track in the
-// album directory. "" means no art was found.
+// resolveAlbumCoverPath delegates to coverart.ResolveAlbumPath; kept as a
+// local alias so the call sites in this file read naturally.
 func resolveAlbumCoverPath(ctx context.Context, q *dbq.Queries, album dbq.Album) string {
-	if album.CoverArtPath != nil && *album.CoverArtPath != "" {
-		if _, err := os.Stat(*album.CoverArtPath); err == nil {
-			return *album.CoverArtPath
-		}
-	}
-	tracks, err := q.ListTracksByAlbum(ctx, dbq.ListTracksByAlbumParams{AlbumID: album.ID})
-	if err != nil || len(tracks) == 0 {
-		return ""
-	}
-	return coverart.FindSidecar(filepath.Dir(tracks[0].FilePath))
+	return coverart.ResolveAlbumPath(ctx, q, album)
 }
 
 func serveImage(w http.ResponseWriter, r *http.Request, path string) {