fix(backend): match official audio by yt-dlp artist metadata, not just channel name

The strict cascade steps required the YouTube channel name to contain the artist, so
official uploads on label channels (BANGTANTV, HYBE LABELS, …) were rejected and the
no-channel fallback grabbed a fan reupload instead (observed: BTS "Pied Piper").

Add yt-dlp's music `artist` field as an OR alternative to the channel check in the
strict steps (two --match-filter args = OR). A metadata dump of "BTS Pied Piper"
confirmed only the official upload has artist=BTS (fan/live/lyric reuploads: NA), and
it's the #1 result — so it now passes strict step 2. channel_is_verified was rejected
as a signal: lyric videos and fan MVs also run on verified channels.

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

backend/lib/ytdlp.js

@@ -122,6 +122,12 @@ async function downloadTrack({ jobId, artist, rawArtist, albumArtist, title, alb
   const artistFirstWord = (rawArtist || mainArtist).split(/[\s,/]+/)[0];
   const artistChannelPart = (artistFirstWord.length >= 3 && !JP_RE.test(artistFirstWord))
     ? ` & channel ~= (?i)${escRe(artistFirstWord)}` : '';
+  // yt-dlp fills `artist` (music metadata) on YouTube "Art Tracks" — the label-provided official
+  // audio — even when the channel doesn't contain the artist name (BANGTANTV/HYBE/…). Verified with
+  // "BTS Pied Piper": ONLY the official upload had artist=BTS; every fan/live/lyric reupload had
+  // none. Used as an OR alternative to the channel check so those official uploads still pass strict.
+  const artistMetaPart = (artistFirstWord.length >= 3 && !JP_RE.test(artistFirstWord))
+    ? ` & artist ~= (?i)${escRe(artistFirstWord)}` : '';
 
   // Use a space (not " - ") between artist and title: this matches how users type
   // in the YTM search box and ranks the exact official track first. A dash skews
@@ -240,8 +246,15 @@ async function downloadTrack({ jobId, artist, rawArtist, albumArtist, title, alb
     } else {
       durPart = '';
     }
-    return ['--match-filter',
-      `${durPart}!is_live${titleKwPart}${strict ? artistChannelPart : ''}${variant ? variantPart : ''}`];
+    const base = `${durPart}!is_live${titleKwPart}${variant ? variantPart : ''}`;
+    // Non-strict, or no artist constraint (short / Japanese name): single filter.
+    if (!strict || !artistChannelPart) return ['--match-filter', base];
+    // Strict: pass if the channel name matches the artist OR yt-dlp's music `artist` metadata does
+    // (catches official label-channel uploads like BANGTANTV). Two --match-filter args = OR in yt-dlp.
+    return [
+      '--match-filter', `${base}${artistChannelPart}`,
+      '--match-filter', `${base}${artistMetaPart}`,
+    ];
   };
 
   const PI_FILTERED    = ['--playlist-items', '1-5',  '--max-downloads', '1'];