fix: remove Spotify credential requirement — no API keys needed

spotdl (SpotAPI) works without developer credentials. Remove the
health-check warning, credential error UI, and Client Credentials
fallback that were blocking the importer.

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

backend/novasonic-server.js

@@ -224,29 +224,7 @@ async function fetchSpotifyToken() {
     _spSecretCache = null; // force secret re-extraction next time
   }
 
-  // Approach 2: Client Credentials fallback (requires SPOTIFY_CLIENT_ID/SECRET in env)
-  if (!CONFIG.spotifyClientId || !CONFIG.spotifyClientSecret) {
-    throw new Error(
-      'Spotify-Token nicht abrufbar (TOTP gescheitert). ' +
-      'SPOTIFY_CLIENT_ID und SPOTIFY_CLIENT_SECRET setzen als Fallback.'
-    );
-  }
-  const basic = Buffer.from(`${CONFIG.spotifyClientId}:${CONFIG.spotifyClientSecret}`).toString('base64');
-  const r = await _fetchTimeout('https://accounts.spotify.com/api/token', {
-    method: 'POST',
-    headers: { 'Authorization': `Basic ${basic}`, 'Content-Type': 'application/x-www-form-urlencoded' },
-    body: 'grant_type=client_credentials',
-  });
-  if (!r.ok) {
-    const errBody = await r.text().catch(() => '');
-    throw new Error(`Spotify-Token-Fehler (${r.status}): ${errBody.slice(0, 200)}`);
-  }
-  const d = await r.json();
-  if (!d.access_token) throw new Error('Kein access_token von Spotify erhalten');
-  _spTokenCache  = d.access_token;
-  _spTokenExpiry = Date.now() + (d.expires_in * 1000) - 30000;
-  console.log('[spotify] token ok (Client Credentials)');
-  return _spTokenCache;
+  throw new Error('Spotify-Token nicht abrufbar (TOTP gescheitert)');
 }
 
 async function _spApi(path, token) {
@@ -370,9 +348,7 @@ app.get('/api/health', (req, res) => {
     squidQuality: CONFIG.squidQuality,
     activeJobs: activeDownloads,
     queueLength: downloadQueue.length,
-    spotify: {
-      configured: !!(CONFIG.spotifyClientId && CONFIG.spotifyClientSecret),
-    },
+    spotify: { configured: true },
   });
 });