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>

tools/snapshot-stopped.js

@@ -21,7 +21,6 @@ function copy(source, relative) {
 }
 for (const name of ['db.json', 'scan-cache.json', 'catalog.json', 'collection-covers', 'runtime-reader.json']) copy(path.join(data, name), path.join('data', name));
 for (const name of ['queue.json', 'runtime.json', 'daemon.lock', 'supervisor.lock']) copy(path.join(data, 'scraper', name), path.join('data', 'scraper', name));
-for (const name of ['state.json', 'runtime.json']) copy(path.join(data, 'sentinel', name), path.join('data', 'sentinel', name));
 for (const name of fs.readdirSync(library)) {
   copy(path.join(library, name, name.startsWith('.mcscraper-work-') ? 'job.json' : 'manga.json'), path.join('library', name, name.startsWith('.mcscraper-work-') ? 'job.json' : 'manga.json'));
 }