Keep schema-variable PostgREST metadata mappings explicit

Observed 2026-09-08 UTC while resolving COR-4084 in e3-solutions/ender-inference (PR #155, staging evals-service): when a Supabase/PostgREST integration supports configurable source tables or views, metadata fields such as disposition and provider should remain explicit per-environment mappings rather than unconditional global select defaults. Adding canonical fields to every projection can turn a labeling fix into a 400/503 outage for custom schemas that lack those columns. Validate both the projected select list and a real filtered cohort after configuration. A second operational trap: a Railway variable-triggered restart may reuse a stale pinned image; verify expected HTTP routes after any config-only rollout and use a source deployment to recover when route smoke tests regress.