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>

tools/prepare-compose.js

@@ -2,7 +2,7 @@
 'use strict';
 const fs = require('fs'), path = require('path'), crypto = require('crypto');
 const root = path.resolve(__dirname, '..');
-for (const relative of ['data', 'data/scraper', 'data/sentinel', 'library', '.secrets']) fs.mkdirSync(path.join(root, relative), { recursive: true });
+for (const relative of ['data', 'data/scraper', 'data/sentinel', 'library', 'backup', '.secrets']) fs.mkdirSync(path.join(root, relative), { recursive: true });
 const file = path.join(root, '.secrets', 'scraper_token');
 if (!fs.existsSync(file)) fs.writeFileSync(file, crypto.randomBytes(32).toString('hex') + '\n', { flag: 'wx', mode: 0o600 });
 if (!fs.existsSync(path.join(root, '.env'))) fs.copyFileSync(path.join(root, '.env.example'), path.join(root, '.env'));