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>

docs/settings-navigation.md

@@ -59,6 +59,10 @@ download times by manga/source, and a collapsed recent-check log. Successful
 no-change nightly checks update this summary and leave the queue. Sources owns
 the per-hoster request and chapter delay settings.
 
+Service health also holds the **Backups** card: built-in backup status (last run,
+size, duration, error), **Back up now**, and the daily schedule and retention form,
+whose draft survives the page's polling. See [backup.md](backup.md).
+
 Reading-time collection controls, export and deletion live under Reading time &
 privacy; reader layout controls stay under Reading. Rescan controls and user
 administration are separate. Server & library describes existing maintenance,