Harden RC7 API response contracts

docs/API_CONTRACTS.md

@@ -9,6 +9,12 @@ This document freezes the first API shape. Codex may add implementation details
 - GeoJSON accepted for geometries where possible.
 - Long processing tasks return a job or analysis run record instead of blocking.
 - Error responses use the shared `ApiError` schema.
+- Every successful JSON endpoint has a concrete Pydantic response model and
+  uses the canonical `{"data": ...}` envelope. Readiness runs an OpenAPI audit
+  that rejects free-form dictionary responses and envelope drift.
+- The only successful non-envelope responses are `/health`, `/health/live`,
+  `/health/ready`, the four documented persisted-raster PNG endpoints and the
+  export artifact download endpoint.
 
 ## Shared schemas