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/api/cast_token.go

@@ -33,28 +33,18 @@ type castTokenResponse struct {
 	Title string `json:"title"`
 }
 
-// mimeForFormat maps the tracks.file_format column to an HTTP audio
-// MIME type. Sonos requires the protocolInfo MIME on DIDL-Lite to match
-// what the URL actually serves; "audio/*" wildcard is silently rejected.
-// Unknown formats fall back to audio/mpeg — most Sonos firmware probes
-// the URL anyway and recovers from a small MIME mismatch.
+// mimeForFormat returns the audio MIME type for a cast (Sonos/UPnP) URL.
+// Wraps the canonical audioContentType lookup in media.go and overrides
+// the unknown-format fallback to audio/mpeg, because Sonos rejects
+// DIDL-Lite with protocolInfo=application/octet-stream (the browser
+// fallback) -- most Sonos firmware probes the URL anyway and recovers
+// from a small MIME mismatch.
 func mimeForFormat(format string) string {
-	switch strings.ToLower(strings.TrimSpace(format)) {
-	case "mp3", "mpeg":
-		return "audio/mpeg"
-	case "flac":
-		return "audio/flac"
-	case "aac", "m4a", "mp4":
-		return "audio/mp4"
-	case "ogg", "vorbis":
-		return "audio/ogg"
-	case "opus":
-		return "audio/opus"
-	case "wav", "wave":
-		return "audio/wav"
-	default:
+	mime := audioContentType(format)
+	if mime == "application/octet-stream" {
 		return "audio/mpeg"
 	}
+	return mime
 }
 
 // extForFormat maps the tracks.file_format column to a path-safe file
@@ -144,8 +134,7 @@ func (h *handlers) handleCastStreamToken(w http.ResponseWriter, r *http.Request)
 	// Include the file extension in the path so Sonos's URL probe sees a
 	// recognizable audio file. Without it, Sonos reports TrackDuration=0
 	// and seeks past 0s land "after the end" -> early track-skip.
-	url := scheme + "://" + host + "/api/tracks/" + req.TrackID +
-		"/stream." + extForFormat(track.FileFormat) +
+	url := scheme + "://" + host + streamURLWithExt(trackUUID, extForFormat(track.FileFormat)) +
 		"?token=" + token + "&exp=" + strconv.FormatInt(exp, 10)
 
 	writeJSON(w, http.StatusOK, castTokenResponse{