feat(backend): stabilize draft editor flow and complete smoke regression baseline

- add editor entry flow with editor context and ensure-draft bootstrap
- add draft summary read model and single-record draft read endpoints
- add typed draft, edit and publish conflicts with validation errors
- add pricing diagnostics and publish readiness endpoints
- fix Decimal serialization in seat price and test preview flows
- harden draft lifecycle guards for published vs draft current version
- update API map and smoke regression checklist
- add backend README and smoke regression script

backend/app/api/routes/structure.py

@@ -244,7 +244,7 @@ async def get_effective_seat_price(scheme_id: str, seat_id: str, role: str = Dep
         matched_rule_level=matched_rule_level,
         matched_target_ref=rule["target_ref"],
         pricing_category_id=rule["pricing_category_id"],
-        amount=rule["amount"],
+        amount=str(rule["amount"]),
         currency=rule["currency"],
     )