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/metadata.js

@@ -0,0 +1,143 @@
+const https = require('https');
+const path  = require('path');
+const fs    = require('fs');
+
+function sanitize(s) {
+  return (s || 'Unknown').replace(/[\\/:*?"<>|]/g, '_').trim().slice(0, 100);
+}
+
+function navidromeArtist(name) {
+  if (!name) return name;
+  return name
+    .replace(/\s+feat\.?\s+/gi, ' / ')
+    .replace(/\s+ft\.?\s+/gi, ' / ')
+    .replace(/\s+featuring\s+/gi, ' / ')
+    .replace(/,\s+/g, ' / ');
+}
+
+// Download hi-res iTunes artwork (1000x1000) to cover.jpg, return path or null
+async function downloadArtwork(url, dir) {
+  if (!url) return null;
+  const dest  = path.join(dir, 'cover.jpg');
+  const hiRes = url.replace(/\d+x\d+bb\./, '1000x1000bb.');
+  return new Promise(resolve => {
+    const tryUrl = (u, fallback) => {
+      const file = fs.createWriteStream(dest);
+      https.get(u, res => {
+        if (res.statusCode !== 200) {
+          res.resume();
+          file.close();
+          try { fs.unlinkSync(dest); } catch {}
+          if (fallback) tryUrl(fallback, null);
+          else resolve(null);
+          return;
+        }
+        res.pipe(file);
+        file.on('finish', () => resolve(dest));
+        file.on('error',  () => resolve(null));
+      }).on('error', () => {
+        if (fallback) tryUrl(fallback, null);
+        else resolve(null);
+      });
+    };
+    tryUrl(hiRes, url !== hiRes ? url : null);
+  });
+}
+
+const AUDIO_EXTS = ['mp3', 'flac', 'opus', 'm4a', 'ogg', 'wav'];
+
+function findExistingTrack(dir, trackPrefix, safeTitle) {
+  return AUDIO_EXTS
+    .map(ext => path.join(dir, `${trackPrefix}${safeTitle}.${ext}`))
+    .find(f => fs.existsSync(f)) || null;
+}
+
+// Fetch plain + synced lyrics from LRCLIB
+async function fetchLyrics(artist, title, album) {
+  try {
+    const p = new URLSearchParams({ artist_name: artist, track_name: title });
+    if (album) p.set('album_name', album);
+    const r = await fetch(`https://lrclib.net/api/get?${p}`, {
+      headers: { 'User-Agent': 'NovaSonic/1.0' },
+      signal: AbortSignal.timeout(8000),
+    });
+    if (!r.ok) return null;
+    const d = await r.json();
+    return { plain: d.plainLyrics || null, synced: d.syncedLyrics || null };
+  } catch { return null; }
+}
+
+// iTunes track lookup — fills in album, year, trackNum, totalTracks, genre when missing
+async function lookupItunesTrackMeta(artist, title) {
+  try {
+    const url = `https://itunes.apple.com/search?term=${encodeURIComponent(artist + ' ' + title)}&media=music&entity=musicTrack&limit=5&country=us&explicit=Yes`;
+    const results = await new Promise((resolve, reject) => {
+      https.get(url, res => {
+        let d = ''; res.on('data', c => d += c);
+        res.on('end', () => { try { resolve(JSON.parse(d).results || []); } catch { reject(new Error('parse')); } });
+      }).on('error', reject);
+    });
+    const norm = s => s?.toLowerCase().trim() || '';
+    const match = results.find(t =>
+      norm(t.trackName) === norm(title) &&
+      norm(t.artistName).split(' ')[0] === norm(artist).split(' ')[0]
+    );
+    if (!match) return {};
+    return {
+      album:       match.collectionName || '',
+      year:        match.releaseDate ? new Date(match.releaseDate).getFullYear() : undefined,
+      trackNum:    match.trackNumber   || undefined,
+      totalTracks: match.trackCount    || undefined,
+      discNum:     match.discNumber    || undefined,
+      discTotal:   match.discCount     || undefined,
+      genre:       match.primaryGenreName || undefined,
+      artworkUrl:  (match.artworkUrl100 || '').replace('100x100bb', '1000x1000bb'),
+      durationMs:  match.trackTimeMillis  || undefined,
+    };
+  } catch { return {}; }
+}
+
+// Fetch individual tracks for an iTunes collectionId
+async function fetchItunesTracks(collectionId) {
+  return new Promise((resolve, reject) => {
+    const url = `https://itunes.apple.com/lookup?id=${collectionId}&entity=song&explicit=Yes`;
+    https.get(url, res => {
+      let data = '';
+      res.on('data', d => data += d);
+      res.on('end', () => {
+        try {
+          const parsed = JSON.parse(data);
+          resolve((parsed.results || []).filter(r => r.wrapperType === 'track'));
+        } catch { reject(new Error('iTunes API parse error')); }
+      });
+    }).on('error', reject);
+  });
+}
+
+// Search iTunes for album tracks by artist + album name (fallback when no collectionId)
+async function searchItunesAlbumTracks(artist, album) {
+  return new Promise(resolve => {
+    const q = encodeURIComponent(`${artist} ${album}`);
+    https.get(`https://itunes.apple.com/search?term=${q}&media=music&entity=album&limit=5&country=us&explicit=Yes`, res => {
+      let d = '';
+      res.on('data', c => d += c);
+      res.on('end', () => {
+        try {
+          const results = JSON.parse(d).results || [];
+          const match = results.find(r =>
+            r.collectionName?.toLowerCase().includes(album.toLowerCase()) ||
+            r.artistName?.toLowerCase().includes(artist.toLowerCase())
+          );
+          if (match) fetchItunesTracks(match.collectionId).then(resolve).catch(() => resolve([]));
+          else resolve([]);
+        } catch { resolve([]); }
+      });
+    }).on('error', () => resolve([]));
+  });
+}
+
+module.exports = {
+  sanitize, navidromeArtist,
+  downloadArtwork, AUDIO_EXTS, findExistingTrack,
+  fetchLyrics, lookupItunesTrackMeta, fetchItunesTracks, searchItunesAlbumTracks,
+};