fix: full stderr logging per-line, scan embed page for GQL hash

- Log each Python stderr line separately (no 500-char truncation) so
  we can see exactly what's happening in the bundle scan
- Try embed page (open.spotify.com/embed/...) first for GQL hashes —
  focused app, fewer bundles, GQL chunks directly referenced
- Suppress noisy bundle-fetch errors to keep logs readable

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

backend/novasonic-server.js

@@ -359,7 +359,7 @@ async function _resolveSpotifyViaSpotdl(url) {
   const cleanUrl = url.split('?')[0];
   return new Promise((resolve, reject) => {
     execFile('python3', [scriptPath, cleanUrl], { timeout: 90000, maxBuffer: 10 * 1024 * 1024 }, (err, stdout, stderr) => {
-      if (stderr?.trim()) console.log('[spotdl-py]', stderr.trim().slice(0, 500));
+      if (stderr?.trim()) stderr.trim().split('\n').forEach(l => console.log('[py]', l));
       // Always try to parse stdout first — script outputs JSON even on error
       const out = stdout?.trim();
       if (out) {