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>

.gitignore

@@ -3,6 +3,8 @@
 /data/
 # the manga library: demo content is generated by `node tools/make-dummy.js`, a real one is yours alone
 /library/
+# built-in backups (BACKUP_PATH default)
+/backup/
 
 # --- Node ---
 node_modules/