Auto-Sync manager: add LB / Last.fm / SoulSync Discovery / iTunes labels

``autoSyncSourceLabel`` was missing entries for the post-Phase-0
sources, so any mirrored playlists with ``source='listenbrainz'``
or ``'lastfm'`` rendered their raw lowercase identifier in the
sidebar's group heading instead of a friendly brand label. Added
the four newer sources. Also added ``itunes_link`` which the iTunes
link tab has been able to create for a few releases now.

Cosmetic only — the existing ``autoSyncCanSchedulePlaylist`` gate
already accepts everything except ``file`` and ``beatport``, so
these sources were always schedulable; the group heading just had
no human label.

webui/static/auto-sync.js

@@ -77,6 +77,10 @@ function autoSyncSourceLabel(source) {
         qobuz: 'Qobuz',
         beatport: 'Beatport',
         file: 'File Imports',
+        itunes_link: 'iTunes Link',
+        listenbrainz: 'ListenBrainz',
+        lastfm: 'Last.fm Radio',
+        soulsync_discovery: 'SoulSync Discovery',
     };
     return labels[source] || source || 'Other';
 }