DB, schema and config readiness

Runs the readiness checks in order and stops at the first failure: database (the database answers), schema (the applied migrations include every migration of this build) and config (required settings and limits are valid). Use it as a readiness probe: a 503 means do not route traffic here. The execution backend is not checked. From the moment the process starts shutting down it answers 503 too.

Auth: public; no credential.