feat: SoundCloud downloader + rename page to Song Importer

- New src/soundcloud.js: resolves SoundCloud URLs via yt-dlp --dump-single-json,
  renders track/playlist preview with per-track download buttons
- Backend POST /api/soundcloud/resolve: extracts metadata (title, artist, genre,
  artwork, duration) directly from SoundCloud — no iTunes lookup needed
- ytdlp.js: sourceUrl param — if set, downloads directly from URL instead of
  ytsearch cascade (used for SoundCloud tracks)
- Spotify importer page renamed to Song Importer, now contains both
  Spotify and SoundCloud sections in one view

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

backend/lib/ytdlp.js

@@ -47,7 +47,7 @@ function buildVariantFilter(title) {
   return parts.length ? parts.join(' & ') : '';
 }
 
-async function downloadTrack({ jobId, artist, albumArtist, title, album, year, trackNum, totalTracks, discNum, discTotal, compilation, genre, artworkUrl, explicit, outputDir, durationMs }) {
+async function downloadTrack({ jobId, artist, albumArtist, title, album, year, trackNum, totalTracks, discNum, discTotal, compilation, genre, artworkUrl, explicit, outputDir, durationMs, sourceUrl }) {
   const mainArtist = albumArtist || artist;
   const safeArtist = sanitize(mainArtist.split(',')[0].trim());
   const safeAlbum  = sanitize(album || 'Singles');
@@ -109,6 +109,15 @@ async function downloadTrack({ jobId, artist, albumArtist, title, album, year, t
   const expectedAudio = path.join(dir, `${trackPrefix}${safeTitle}.${CONFIG.audioFmt}`);
   const hasAudio = () => fs.existsSync(expectedAudio);
 
+  // Direct URL download (e.g. SoundCloud) — skip the ytsearch cascade entirely
+  if (sourceUrl) {
+    log(jobId, `🔗 Direkter Download: ${sourceUrl}`);
+    await spawnYtdlp([sourceUrl, '--no-playlist', ...baseArgs]);
+    const audioFile = hasAudio() ? expectedAudio : null;
+    if (!audioFile) { log(jobId, '⚠️  Download fehlgeschlagen'); return { success: true, dir }; }
+    return embedMetadata({ jobId, audioFile, dir, trackPrefix, safeTitle, artist, mainArtist, album, year, trackNum, totalTracks, discNum, discTotal, compilation, genre, artworkUrl, explicit, title });
+  }
+
   // Fetch YTM search result URLs as a flat list (no channel expansion) and return only
   // actual video URLs (watch?v=…). This prevents yt-dlp from following an artist channel
   // result and treating its uploads as the search result set.