add disable postgres check flag on healthcheck

[tsuzukayama]

Add internalApi.healthcheck.skipPostgresCheck flag to disable Postgres checking on healthcheck endpoint. Doing this to enable Postgres maintenance. Maestro core functionality should still work even though the DB is offline (Scheduler Management will be down).

---

Note

Adds a config flag to optionally skip the Postgres check in /readyz and updates the handler/logging and responses accordingly.

• Internal API:
  • Readiness (/readyz):
    • Add config-gated Postgres check via internalApi.healthcheck.skipPostgresCheck (when true, skip DB connectivity check).
    • Update handler to accept config, apply skip logic, and set Content-Type consistently.
    • Return 503 Service Unavailable with body when unhealthy; log a warning if Postgres check is disabled.
• Config:
  • Add internalApi.healthcheck.skipPostgresCheck: false default in config/config.yaml.

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