feat(health): service_status cache table + repo

lib/db/migrations/014_service_status.sql

@@ -0,0 +1,8 @@
+-- 014_service_status.sql
+CREATE TABLE service_status (
+  service_id  text PRIMARY KEY,
+  status      text NOT NULL CHECK (status IN ('ok','warn','down','unknown')),
+  latency_ms  integer,
+  detail      text,
+  checked_at  timestamptz NOT NULL DEFAULT now()
+);