fix: strip ?si= from Spotify URL, improve spotdl error logging

spotdl doesn't handle tracking params in URLs. Also separate stdout/stderr
in Python script and always try parsing stdout before checking exit code.

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

backend/novasonic-server.js

@@ -379,16 +379,22 @@ app.get('/api/health', (req, res) => {
 async function _resolveSpotifyViaSpotdl(url) {
   const { execFile } = require('child_process');
   const scriptPath = path.join(__dirname, 'spotify_resolve.py');
+  // Strip ?si= and other tracking params — spotdl doesn't handle them
+  const cleanUrl = url.split('?')[0];
   return new Promise((resolve, reject) => {
-    execFile('python3', [scriptPath, url], { timeout: 90000, maxBuffer: 10 * 1024 * 1024 }, (err, stdout, stderr) => {
-      if (err) return reject(new Error(`spotdl: ${stderr?.trim() || err.message}`));
-      try {
-        const data = JSON.parse(stdout.trim());
-        if (data.error) return reject(new Error(data.error));
-        resolve(data);
-      } catch {
-        reject(new Error(`spotdl: ungültige Ausgabe — ${stdout.slice(0, 200)}`));
+    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));
+      // Always try to parse stdout first — script outputs JSON even on error
+      const out = stdout?.trim();
+      if (out) {
+        try {
+          const data = JSON.parse(out);
+          if (data.error) return reject(new Error(`spotdl: ${data.error}`));
+          return resolve(data);
+        } catch { /* not JSON, fall through */ }
       }
+      if (err) return reject(new Error(`spotdl exited ${err.code}: ${stderr?.trim() || out || err.message}`));
+      reject(new Error(`spotdl: keine Ausgabe`));
     });
   });
 }