Liveness + readiness probe.

Always returns 200 if the process is up — Fly's health check uses this to decide whether the machine is alive. The db field reports "up" when a trivial SELECT 1 round-trips successfully against the primary database, "down" otherwise. DB-down does NOT trip the 200 → 503 boundary (per liveness semantics: process-up ≠ deps-up, and machine restart wouldn't fix a downstream DB outage).