Implement comprehensive health checks

[chitcommit]

Description\nAdd health checks for all services, bindings, and external dependencies.\n\nTasks\n- [ ] Service-level health endpoints\n- [ ] Database connectivity checks\n- [ ] AI binding validation\n- [ ] KV/R2 availability\n- [ ] External API health (ChittyID, Registry)\n\nOutput\n- /health/detailed endpoint\n- Prometheus-compatible metrics\n