feat(web): download the Android APK from Settings

Backend serves a staged APK so co-users self-serve the latest build:
- GET /api/apk streams APK_PATH (default /data/novasonic.apk) with the
  android package content-type + attachment disposition; 404 if absent.
- GET /api/apk/info reports availability, size, mtime, and an optional
  version from /data/novasonic.version.txt.
- Settings gets a "🤖 Android App" section that appears only when an APK is
  present and shows version/size/date.

Maintainer updates the APK with:
  docker cp app-release.apk novasonic-backend:/data/novasonic.apk

Android source is already tracked; only build artifacts stay out of git.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/novasonic-server.js

@@ -466,6 +466,33 @@ app.get('/api/events-debug/:artist', (req, res) => {
   }).on('error', e => res.json({ error: e.message }));
 });
 
+// ── NovaSonic Android APK distribution ──────────────────────────────────────
+// The web app offers a "download the Android app" button that points here, so co-users always get
+// the latest APK without the maintainer hand-delivering it. Drop the latest release build into the
+// writable data volume (APK_PATH, default /data/novasonic.apk):
+//   docker cp app-release.apk novasonic-backend:/data/novasonic.apk
+// Optional: a sidecar /data/novasonic.version.txt with a version string (e.g. "2.2") shown in the UI.
+const APK_PATH     = process.env.APK_PATH || path.join(CONFIG.dataPath, 'novasonic.apk');
+const APK_VER_PATH = path.join(CONFIG.dataPath, 'novasonic.version.txt');
+
+app.get('/api/apk/info', (req, res) => {
+  fs.stat(APK_PATH, (err, st) => {
+    if (err) return res.json({ available: false });
+    let version = null;
+    try { version = fs.readFileSync(APK_VER_PATH, 'utf8').trim() || null; } catch {}
+    res.json({ available: true, size: st.size, updatedAt: st.mtimeMs, version });
+  });
+});
+
+app.get('/api/apk', (req, res) => {
+  fs.stat(APK_PATH, (err) => {
+    if (err) return res.status(404).json({ error: 'Keine APK hinterlegt' });
+    res.setHeader('Content-Type', 'application/vnd.android.package-archive');
+    res.setHeader('Content-Disposition', 'attachment; filename="NovaSonic.apk"');
+    res.sendFile(APK_PATH);
+  });
+});
+
 app.get('/api/health', (req, res) => {
   res.json({
     status: 'ok', ytdlpVersion,