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/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)