Service health check

Health check for the API. Returns 200 only when every dependency the service cannot work without (today, its database) has been proven reachable within the last few seconds; otherwise 503 with the same payload, status of degraded, and a dependencies.checks entry naming what failed and which category of failure it was. Public, with no authentication required, so agents, uptime monitors, and load balancers can reach it without a credential. Treat any non-200 as the API being unable to serve authenticated requests. The result is cached briefly and shared across callers, so polling it frequently costs the service nothing.