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{

internal/api/convert.go

@@ -75,6 +75,15 @@ func streamURL(trackID pgtype.UUID) string {
 	return "/api/tracks/" + uuidToString(trackID) + "/stream"
 }
 
+// streamURLWithExt returns the extension-bearing stream URL used by UPnP
+// cast tokens. Sonos's URL probe gates duration detection on a recognizable
+// audio file extension; the bare `/stream` shape reports TrackDuration=0
+// and breaks seek/auto-advance. The bare /stream route stays mounted as an
+// alias for legacy / web / Subsonic clients. See task #610.
+func streamURLWithExt(trackID pgtype.UUID, ext string) string {
+	return streamURL(trackID) + "." + ext
+}
+
 // 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,

internal/api/media.go

@@ -22,46 +22,42 @@ import (
 	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
 )
 
-// resolveAlbumCoverPath returns the filesystem path to the album's cover art.
-// It prefers an explicit cover_art_path (set by the scanner in a future
-// milestone) and falls back to a sidecar next to the first track in the
-// album's 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)
 }
 
 // audioContentType maps the short file_format recorded on tracks (mp3, flac,
 // ogg, opus, m4a, aac, wav) to a MIME type for the Content-Type header.
-// Unknown formats fall back to octet-stream so the browser downloads them
-// rather than attempting to decode.
+// This is the canonical table; both the browser stream endpoint and the
+// UPnP cast token URL builder consult it. Unknown formats fall back to
+// octet-stream so the browser downloads them rather than attempting to
+// decode -- cast_token.go applies its own audio/mpeg fallback for Sonos.
+//
+// Aliases (mpeg/vorbis/wave) cover historical / alternate format spellings
+// that have shown up in track rows. The trim+lowercase normalization makes
+// the lookup permissive to whatever a scanner happened to write.
 //
 // Divergences from internal/subsonic/types.go's contentTypeForFormat are
-// intentional: opus→audio/ogg (library .opus files are Ogg-encapsulated, so
-// this matches real library contents), aac→audio/aac (raw AAC is ADTS, not
-// MP4, so audio/mp4 would mislead codec sniffers), and there is no "oga" case
-// (we don't record that format). Don't "fix" these to match subsonic.
+// intentional: opus/vorbis→audio/ogg (library .opus / .ogg files are
+// Ogg-encapsulated, so this matches real library contents), aac→audio/aac
+// (raw AAC is ADTS, not MP4, so audio/mp4 would mislead codec sniffers),
+// and there is no "oga" case (we don't record that format). Subsonic is a
+// frozen client contract -- don't "fix" these to match it.
 func audioContentType(format string) string {
-	switch strings.ToLower(format) {
-	case "mp3":
+	switch strings.ToLower(strings.TrimSpace(format)) {
+	case "mp3", "mpeg":
 		return "audio/mpeg"
 	case "flac":
 		return "audio/flac"
-	case "ogg", "opus":
+	case "ogg", "opus", "vorbis":
 		return "audio/ogg"
-	case "m4a":
+	case "m4a", "mp4":
 		return "audio/mp4"
 	case "aac":
 		return "audio/aac"
-	case "wav":
+	case "wav", "wave":
 		return "audio/wav"
 	}
 	return "application/octet-stream"

internal/api/playlists.go

@@ -476,8 +476,8 @@ func playlistDetailToView(d *playlists.PlaylistDetail) playlistDetailView {
 		if t.TrackID != nil {
 			s := uuidToString(*t.TrackID)
 			v.TrackID = &s
-			streamURL := "/api/tracks/" + s + "/stream"
-			v.StreamURL = &streamURL
+			url := streamURL(*t.TrackID)
+			v.StreamURL = &url
 		}
 		if t.AlbumID != nil {
 			s := uuidToString(*t.AlbumID)