feat(backend): harden pricing mutation contract and sync backend docs

- add typed response schemas for pricing write endpoints
- add stale draft version guard for pricing mutations
- unify pricing API contract around expected_scheme_version_id
- update API route map
- add smoke regression checklist for backend routes and artifact flows

backend/docs/smoke-regression.md

@@ -62,16 +62,20 @@ Validate:
 - no 500 on passthrough mode
 - unsupported mode returns 422
 
-## 5. Pricing read model
+## 5. Pricing read / write contract
 
 - GET /api/v1/schemes/{scheme_id}/pricing -> 200
 - GET /api/v1/schemes/{scheme_id}/current/seats/{seat_id}/price -> 200 for priced seat
 - GET /api/v1/schemes/{scheme_id}/test/seats/{seat_id} -> 200 for known seat
+- POST /api/v1/schemes/{scheme_id}/pricing/categories?expected_scheme_version_id={draft_version_id} -> 200 for valid draft
+- POST /api/v1/schemes/{scheme_id}/pricing/categories?expected_scheme_version_id=deadbeef... -> 409 for stale draft
 
 Validate:
 - pricing bundle contains categories and rules arrays
 - effective seat price resolves according to domain priority
 - test seat preview explains selectable / has_price state
+- pricing write responses are stable and typed
+- stale pricing mutation returns `detail.code = stale_draft_version`
 
 ## 6. Draft publish preview
 
@@ -122,6 +126,8 @@ Regression is considered failed if any of the following happen:
 - passthrough display endpoint fails on known-good sample
 - publish preview refresh or cached read returns 500
 - pricing bundle contract changes unexpectedly
+- pricing write contract regresses or stops returning typed payloads
+- stale draft guard stops returning 409 on pricing mutations
 - admin audit/cleanup endpoints fail on healthy environment
 - artifact retention grows without bound for repeated preview refresh on same variant