fix: use original artist name in YTM search and add strict channel filter

Use mainArtist (with special chars) for ytsearch queries so e.g. "N!GHT - Yume"
searches correctly instead of "NGHT - Yume" which YouTube misreads as "night"
and returns ambience videos.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/lib/ytdlp.js

@@ -64,8 +64,10 @@ async function downloadTrack({ jobId, artist, albumArtist, title, album, year, t
   }
 
   const outputTemplate = path.join(dir, `${trackPrefix}${safeTitle}.%(ext)s`);
-  const searchArtist = mainArtist.replace(/[^\p{L}\p{N}\s,&\-]/gu, '').trim();
-  const query = `${searchArtist} - ${title}`;
+  // Use original artist name (with special chars) for all searches — stripping e.g. "N!GHT" → "NGHT"
+  // causes ytsearch to match "night" ambience videos instead of the artist.
+  // spawn() passes args directly without shell interpretation so special chars are safe.
+  const query = `${mainArtist} - ${title}`;
 
   const baseArgs = [
     '--extract-audio',