Add configDbHealthy parameter to OpcUaApplicationHost; wire a DbHealthCache (CanConnectAsync cached 10 s) in Program.cs so /healthz reflects real config-DB reachability instead of the previous always-true default; /healthz now returns 503 on a DB outage unless stale-config cache is warm. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>