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>

README.md

@@ -74,6 +74,7 @@ see administration; personal settings do not poll scraper services. See
 | `SCRAPER_TOKEN` | empty       | Shared daemon token; required when it listens beyond loopback |
 | `SCRAPER_TOKEN_FILE` | empty | Alternative secret file (used in Compose) |
 | `SENTINEL_URL` | `http://127.0.0.1:8092` | Private observation service URL |
+| `BACKUP`        | –           | Existing folder for built-in backups of `DATA`; unset = off. Compose: set `BACKUP_PATH`. Schedule and retention under Settings → Service health ([backup](docs/backup.md)) |
 
 Serve it behind a reverse proxy with HTTPS if it is reachable from the internet. All pages use relative URLs, so a sub-path like `/manga/` works.
 
@@ -113,6 +114,7 @@ whole-series completeness. See the scraper service and library format contracts
 | [docs/architecture.md](docs/architecture.md) | Components, data model, API, security, the reader's layout engine, deployment, design decisions |
 | [docs/local-deployment.md](docs/local-deployment.md) | Current local Compose deployment, Tailscale access, persistent state, checks, restart, and recovery |
 | [docs/settings-navigation.md](docs/settings-navigation.md) | User/Admin settings sections, scraper tabs, routes and UI checks |
+| [docs/backup.md](docs/backup.md) | Built-in scheduled backup of `data/`, what it contains, retention and the restore procedure |
 | [docs/reading-insights-concept.md](docs/reading-insights-concept.md) | Active-time presentation, private summaries, deletion/export controls, and the future journal design |
 | [docs/scraper-guide.md](docs/scraper-guide.md) | Rule book (agent-readable) for scrapers and other tools that write into the library |
 | [scraper/AGENT_RUNBOOK.md](scraper/AGENT_RUNBOOK.md) | Conservative scraper workflow and host-specific procedures for automated agents |