Add built-in backup of the data folder (BL-0044)

The reader owns a scheduled backup into BACKUP (Compose: BACKUP_PATH mounted at
/backup, off when unset). A run pauses the scraper queue through its control
API, waits (bounded) until no job is active, writes a verified tar.gz of DATA
with db.json from memory and scraper/queue.json stored paused, excludes
runtime/lock/pid/temp files, resumes only a queue it paused, then applies
count- or day-based retention to its own archive names only. Schedule,
retention, "Back up now" and last result live in Settings > Service health.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

server.js

@@ -10,6 +10,7 @@
  *   COOKIE_SECURE=1      mark the session cookie Secure (when served over HTTPS)
  *   SCRAPER_URL=http://127.0.0.1:8091   scraper daemon API
  *   SCRAPER_TOKEN=...                    shared daemon token (optional on loopback)
+ *   BACKUP=/backup       folder for built-in DATA backups (unset = backups off)
  */
 const http = require('http'), fs = require('fs'), path = require('path'), crypto = require('crypto');
 const { scanLibrary, scanChangedLibrary, libraryStamps, snapshotIndex, restoreIndex } = require('./library');
@@ -26,6 +27,7 @@ const {
   applyPulse, applyVisit, chapterInsight, eligiblePages, emptyRecord, libraryInsight, mangaInsight, publicSummary,
 } = require('./reading-analytics');
 const { parseSchedule, publicSchedule, loadQuestionBank, createReadingAccess } = require('./reading-access');
+const { BackupService, BackupError } = require('./backup');
 
 const PORT = +process.env.PORT || 8080;
 const LIB = path.resolve(process.env.LIBRARY || 'library');
@@ -65,6 +67,9 @@ function save() { saveTimer = saveTimer || setTimeout(flush, 500); }
 function flush() { clearTimeout(saveTimer); saveTimer = null; writeAtomic(DB_FILE, JSON.stringify(db)); }
 for (const sig of ['SIGINT', 'SIGTERM']) process.on(sig, () => { flush(); process.exit(0); });
 db.readingAccess ||= {};
+// The reader owns backups: it alone mounts all of DATA and holds db.json in memory.
+const backup = new BackupService({ dataRoot: DATA, backupRoot: process.env.BACKUP || '', snapshotDb: () => JSON.stringify(db),
+  scraper: { state: () => scraperRequest('/api/state'), control: action => scraperRequest('/api/control', { action }) } }).start();
 const readingSchedule = parseSchedule(process.env.READING_WINDOWS_JSON || '', process.env.READING_TIMEZONE || process.env.TZ || 'Europe/Berlin');
 const readingAccess = createReadingAccess({ schedule: readingSchedule,
   bank: loadQuestionBank(process.env.READING_QUIZ_CATALOG_PATH || ''),
@@ -377,7 +382,19 @@ const routes = {
     admin(u);
     const sentinel = await fetch(SENTINEL_URL + '/api/state', { headers: SCRAPER_TOKEN ? { authorization: `Bearer ${SCRAPER_TOKEN}` } : {}, signal: AbortSignal.timeout(4000) })
       .then(async r => { if (!r.ok) throw new Error(); return r.json(); }).catch(() => ({ status: 'unreachable' }));
-    return { reader: { ok: !scanState.error, uptime: Math.round(process.uptime()), scan: scanState }, sentinel };
+    return { reader: { ok: !scanState.error, uptime: Math.round(process.uptime()), scan: scanState }, sentinel, backup: backup.publicState() };
+  },
+  'GET /api/admin/backup': ({ u }) => { admin(u); return backup.publicState(); },
+  'POST /api/admin/backup': ({ u, b }) => {
+    admin(u);
+    try {
+      if (b.command === 'settings') return backup.updateSettings(b.settings);
+      if (b.command !== 'run') bad(400, 'Unknown backup command');
+      if (!backup.backupRoot) bad(409, 'Backups are off: set BACKUP to a mounted folder');
+      if (backup.running) bad(409, 'A backup is already running');
+      backup.run('manual').catch(() => {}); // the result lands in last/history
+      return backup.publicState();
+    } catch (e) { if (e instanceof BackupError) bad(e.code, e.message); throw e; }
   },
   'GET /api/auth': () => ({ firstRun: !Object.keys(db.users).length, signup: OPEN_SIGNUP }),