Fold the sentinel into the scraper and drop the sentinel service (BL-0046)

The stall watchdog now runs inside the scraper (ScraperService.checkStalls,
every 30 s from the work loop): same rule as before, one checkpoint-safe
restart, quarantine on the second stall, missing checkpoint is no evidence,
and paused time no longer counts. Stalls are recorded as scraper incidents
(failure.kind 'stall'). The private /api/recovery endpoint is gone.

Dropped with the service: reader/scraper-unreachable incidents (supervisor
and Docker handle crashes; the health page shows "Scraper: unreachable"),
duplicate job-failure incidents and the unused /metrics endpoint.
The stack is now reader + scraper (+ optional peer-gateway).
The supervisor test moved to runtime/test (npm run test:runtime stays).

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

README.md

@@ -20,7 +20,7 @@ Open http://localhost:8080. The first account you create becomes the admin. Try
 node tools/make-dummy.js library
 ```
 
-To run the reader, managed scraper and Sentinel together, use `npm run start:all`. Admins can
+To run the reader and managed scraper together, use `npm run start:all`. Admins can
 then manage the persistent download queue, progress, pauses, retries, and rate limits
 under **Settings → Admin → Scraper**. The reader continues working if the scraper is stopped.
 **Settings → Admin → Collections & metadata** manages grouped tiles, reading order and relationships;
@@ -44,7 +44,7 @@ docker compose up -d --build
 ```
 
 Stop existing native writers before using their data in containers. `docker-compose.yml`
-runs reader, scraper and Sentinel with explicit persistent bind mounts (`library/`,
+runs reader and scraper with explicit persistent bind mounts (`library/`,
 `data/`), file-backed secrets and only a loopback reader port. No named data volume is
 used. See [Compose deployment](docs/compose-deployment.md) for local development, VM
 recommendations, cutover, image releases and backup/rollback. The local Docker stack
@@ -74,7 +74,6 @@ see administration; personal settings do not poll scraper services. See
 | `SCRAPER_URL`   | `http://127.0.0.1:8091` | Private scraper daemon URL |
 | `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.