perf: remote browser CPU / zombie optimization

- Add tini (init: true in compose) to reap orphan Chromium processes
- Reduce screenshot push frequency (active 0.12→0.20s, idle 0.35→1.00s,
  deep idle 1.00→5.00s, backoff 0.60→2.00s)
- Add 5s timeout to screenshot in WebSocket push loop
- close() now wraps context.close() in asyncio.wait_for(10s)
  with browser.close() fallback on timeout
- Two-phase close logging (closing → closed / close_failed)
- Auth-capture sessions evicted after 10min TTL
- shutdown() with timeout protection and logging
- close_ok correctly tracks success through browser fallback path

docker-compose.yml

@@ -24,6 +24,7 @@ services:
       options:
         max-size: "10m"
         max-file: "3"
+    init: true
     healthcheck:
       test: ["CMD", "curl", "-f", "http://localhost:8000/healthz"]
       interval: 30s