feat(backend): harden draft, pricing and publish contracts

- unify typed API errors across draft, pricing and publish flows
- add stale draft and publish-state mutation guards
- add publish readiness contract and guarded publish flow
- add sellability reason codes to test seat preview
- add pricing diagnostics and strengthen snapshot/publish lifecycle consistency

backend/app/services/draft_guard.py

@@ -1,5 +1,3 @@
-from fastapi import HTTPException, status
-
 from app.repositories.scheme_versions import get_current_scheme_version
 from app.repositories.schemes import get_scheme_record_by_scheme_id
 from app.services.api_errors import raise_conflict
@@ -29,9 +27,14 @@ async def get_current_draft_context(
     )
 
     if version.status != "draft" or scheme.status != "draft":
-        raise HTTPException(
-            status_code=status.HTTP_409_CONFLICT,
-            detail="Current scheme version is not editable because it is not in draft state",
+        raise_conflict(
+            code="draft_not_editable",
+            message="Current scheme version is not editable because it is not in draft state",
+            details={
+                "scheme_status": scheme.status,
+                "scheme_version_status": version.status,
+                "actual_scheme_version_id": version.scheme_version_id,
+            },
         )
 
     if expected_scheme_version_id and expected_scheme_version_id != version.scheme_version_id: