feat: split backend into lib/ modules + add squid.wtf (Qobuz) download backend

- backend/lib/config.js  — CONFIG from env (adds DOWNLOAD_BACKEND, SQUID_QUALITY)
- backend/lib/jobs.js    — jobs Map, updateJob(), log()
- backend/lib/metadata.js — iTunes, artwork, lyrics, sanitize helpers
- backend/lib/ytdlp.js   — yt-dlp download cascade + shared embedMetadata()
- backend/lib/squid.js   — ALTCHA PoW solver, Qobuz search/download via squid.wtf
- backend/lib/downloader.js — backend selector: ytdlp | squid | auto (squid-first + fallback)
- novasonic-server.js thinned to Express routes only, imports from lib/
- docker-compose: mount full backend/ dir, install deps outside read-only volume
- .env.example: document DOWNLOAD_BACKEND and SQUID_QUALITY

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

backend/lib/downloader.js

@@ -0,0 +1,60 @@
+const CONFIG       = require('./config');
+const { log, updateJob } = require('./jobs');
+const ytdlp        = require('./ytdlp');
+const squid        = require('./squid');
+
+// Route a single track download to the correct backend, with optional fallback.
+async function downloadTrack(opts) {
+  const backend = CONFIG.downloadBackend;
+
+  if (backend === 'ytdlp') return ytdlp.downloadTrack(opts);
+  if (backend === 'squid') return squid.downloadTrack(opts);
+
+  // 'auto': squid first, yt-dlp as fallback
+  try {
+    const result = await squid.downloadTrack(opts);
+    if (result !== null) return result;
+    // null = not found on Qobuz — fall through to yt-dlp
+    log(opts.jobId, '⚠️  squid.wtf: nicht gefunden — Fallback zu yt-dlp');
+  } catch (e) {
+    log(opts.jobId, `⚠️  squid.wtf Fehler — Fallback zu yt-dlp: ${e.message}`);
+  }
+  return ytdlp.downloadTrack(opts);
+}
+
+// Download all tracks of an album sequentially, updating job progress.
+async function downloadAlbumTracks({ jobId, artist, albumTitle, tracks, artworkUrl }) {
+  const total = tracks.length;
+  log(jobId, `📀 Album "${albumTitle}" — ${total} Tracks`);
+  let done = 0, skipped = 0;
+
+  for (const track of tracks) {
+    try {
+      const result = await downloadTrack({
+        jobId,
+        artist:      track.artistName   || artist,
+        albumArtist: artist,
+        title:       track.trackName    || track.title,
+        album:       albumTitle,
+        year:        track.releaseDate  ? new Date(track.releaseDate).getFullYear() : undefined,
+        trackNum:    track.trackNumber  || undefined,
+        totalTracks: total,
+        discNum:     track.discNumber   || undefined,
+        discTotal:   track.discCount    || undefined,
+        compilation: track.collectionType === 'Compilation' || undefined,
+        genre:       track.primaryGenreName || undefined,
+        artworkUrl:  artworkUrl || track.artworkUrl100,
+        explicit:    track.trackExplicitness === 'explicit',
+        outputDir:   CONFIG.musicPath,
+        durationMs:  track.trackTimeMillis || undefined,
+      });
+      if (result?.skipped) skipped++; else done++;
+      updateJob(jobId, { progress: Math.round(((done + skipped) / total) * 90) });
+    } catch (e) {
+      log(jobId, `⚠️  Fehler bei "${track.trackName || track.title}": ${e.message}`);
+    }
+  }
+  return { done, skipped };
+}
+
+module.exports = { downloadTrack, downloadAlbumTracks };