Fix L2 deployment: healthcheck, DB deadlock, CI image resolution

- Add /health endpoint (returns 200, skips auth middleware)
- Healthcheck now hits /health instead of / (which 302s to OAuth)
- Advisory lock in db.init_pool() prevents deadlock when 4 uvicorn
  workers race to run schema DDL
- CI: --resolve-image always on docker stack deploy to force re-pull

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

l2/docker-compose.yml

@@ -60,7 +60,7 @@ services:
       - OAUTH_LOGOUT_URL=https://account.rose-ash.com/auth/sso-logout/
       # DATABASE_URL, ARTDAG_DOMAIN, ARTDAG_USER, JWT_SECRET, SECRET_KEY from .env file
     healthcheck:
-      test: ["CMD", "python", "-c", "import urllib.request; urllib.request.urlopen('http://localhost:8200/')"]
+      test: ["CMD", "python", "-c", "import urllib.request; urllib.request.urlopen('http://localhost:8200/health')"]
       interval: 10s
       timeout: 5s
       retries: 3