Fix healthcheck + external URL in silent auth middleware

- Add /health endpoint, update healthcheck to use it
- Use configured base URL instead of internal Docker URL
- Add /health to skip prefixes for silent auth

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

docker-compose.yml

@@ -76,7 +76,7 @@ services:
       # DATABASE_URL, ADMIN_TOKEN, ARTDAG_CLUSTER_KEY,
       # L2_SERVER, L2_DOMAIN, IPFS_GATEWAY_URL from .env file
     healthcheck:
-      test: ["CMD", "python", "-c", "import urllib.request; urllib.request.urlopen('http://localhost:8100/')"]
+      test: ["CMD", "python", "-c", "import urllib.request; urllib.request.urlopen('http://localhost:8100/health')"]
       interval: 10s
       timeout: 5s
       retries: 3