revert: drop the YouTube cookies/PO-token/proxy workarounds (back to pre-saga state)

None of it got downloads working on the flagged datacenter IP (web_music media
403, then 429 rate-limiting), so revert the whole lot to keep things clean:
- config.js: remove potProviderUrl + ytdlpProxy
- ytdlp.js: remove potArgs/proxyArgs; player_client back to web_music,default
- novasonic-server.js: remove pot/proxy args from importer resolve
- docker-compose.yml: remove the bgutil-provider service + backend env/plugin/depends_on
- CHANGELOG: drop the [2.2] section

Kept the one genuine fix: a failed direct-link download now reports failure
(moved its note into [2.1]).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/lib/ytdlp.js

@@ -134,18 +134,6 @@ async function downloadTrack({ jobId, artist, rawArtist, albumArtist, title, alb
   // YTM ranking toward a same-named artist's full discography.
   const query = `${ytmArtist} ${searchTitle}`;
 
-  // YouTube 403s media downloads from datacenter IPs without a PO token. When a provider URL is
-  // configured, point the bgutil-ytdlp-pot-provider plugin at it so yt-dlp fetches a token. Empty
-  // config → no arg → unchanged behaviour.
-  const potArgs = () =>
-    CONFIG.potProviderUrl
-      ? ['--extractor-args', `youtubepot-bgutilhttp:base_url=${CONFIG.potProviderUrl}`]
-      : [];
-
-  // Optional proxy — the durable fix when a datacenter IP stays flagged (403/429) even with a PO
-  // token. Route yt-dlp through a residential/rotating proxy via YTDLP_PROXY. Empty → not used.
-  const proxyArgs = () => (CONFIG.ytdlpProxy ? ['--proxy', CONFIG.ytdlpProxy] : []);
-
   const baseArgs = [
     '--extract-audio',
     '--audio-format',   CONFIG.audioFmt,
@@ -156,13 +144,10 @@ async function downloadTrack({ jobId, artist, rawArtist, albumArtist, title, alb
     '--newline',
     // Hard upper size limit — prevents saving a wrong 28-min video as a ~52 MB mp3
     '--max-filesize',   '30M',
-    // For the DOWNLOAD, prefer the default clients (their media URLs go through with a PO token from
-    // a datacenter IP) and keep web_music only as a fallback for Art Tracks that only it can extract.
-    // web_music's own media URLs 403 from datacenter IPs even with a valid PO token. (The YTM *search*
-    // in getYtmVideoUrls still leads with web_music, which is where Art-Track discovery matters.)
-    '--extractor-args', 'youtube:player_client=default,web_music',
-    ...potArgs(),
-    ...proxyArgs(),
+    // Use the web_music player client (falling back to default) so YouTube Music
+    // "Art Track" / topic uploads are reachable. The android_vr client returns
+    // "This video is not available" for many YTM tracks that play fine in the browser.
+    '--extractor-args', 'youtube:player_client=web_music,default',
   ];
 
   const spawnYtdlp = (args) => new Promise((resolve, reject) => {
@@ -235,8 +220,6 @@ async function downloadTrack({ jobId, artist, rawArtist, albumArtist, title, alb
       // generic YouTube search and returns unrelated videos instead of the
       // native Songs shelf (where the actual artist track lives).
       '--extractor-args', 'youtube:player_client=web_music,default',
-      ...potArgs(),
-      ...proxyArgs(),
     ], { env: { ...process.env, PYTHONUNBUFFERED: '1' } });
     proc.stdout.on('data', d => {
       const text = d.toString();