Investigate persistent PostgREST 503 PGRST002 on Mars Data API

PostgREST Data API returning 503 PGRST002 (schema-cache introspection fault) on every /rest/v1/* GET across the visible Supabase log window (hours), predates MARS-416 migration and is unrelated to it. /auth/v1/* healthy throughout (200s from real production traffic), authenticator role has live idle DB connections — reads as a genuine sustained fault, not a deliberate Data-API-disabled toggle (disabled would 404 at gateway, not proxy to this PostgREST error). No Management-API/dashboard access from agent side to confirm the Data API on/off setting directly — needs Supabase Studio > Settings > Data API check to settle definitively. Flagged by db-mars-cc during MARS-416 closure verification; audit-mars-ca called it a separate incident, not evidence either way about the grant fix.