refactor(api): unify typed error contract across draft pricing and publish flows

standardize typed error responses across draft, pricing and publish endpoints

reduce contract drift between related flows
keep client-side handling more predictable and consistent

backend/app/services/editor_validation.py

@@ -1,24 +1,19 @@
 from __future__ import annotations
 
-from fastapi import HTTPException, status
-
 from app.repositories.scheme_groups import list_scheme_version_groups
 from app.repositories.scheme_seats import list_scheme_version_seats
 from app.repositories.scheme_sectors import list_scheme_version_sectors
+from app.services.api_errors import raise_unprocessable
 
 
 def _raise_uniqueness_error(message: str, detail: dict | None = None) -> None:
-    raise HTTPException(
-        status_code=status.HTTP_422_UNPROCESSABLE_ENTITY,
-        detail=detail or {"code": "editor_uniqueness_error", "message": message},
-    )
+    payload = detail or {"code": "editor_uniqueness_error", "message": message}
+    raise_unprocessable(**payload)
 
 
 def _raise_reference_error(message: str, detail: dict | None = None) -> None:
-    raise HTTPException(
-        status_code=status.HTTP_422_UNPROCESSABLE_ENTITY,
-        detail=detail or {"code": "editor_reference_error", "message": message},
-    )
+    payload = detail or {"code": "editor_reference_error", "message": message}
+    raise_unprocessable(**payload)
 
 
 async def validate_single_seat_patch_uniqueness(