feat(backend): add publish readiness endpoint and enforce publish gate contract

add backend endpoint for publish readiness checks

enforce publish gate contract before version publication
make publish preconditions explicit and consistent for clients

backend/app/schemas/publish_readiness.py

@@ -0,0 +1,11 @@
+from pydantic import BaseModel
+
+
+class PublishReadinessResponse(BaseModel):
+    scheme_id: str
+    scheme_version_id: str
+    status: str
+    validation_summary: dict
+    pricing_coverage: dict
+    snapshot: dict
+    readiness: dict