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/remap_service.py

@@ -1,11 +1,10 @@
 from __future__ import annotations
 
-from fastapi import HTTPException, status
-
 from app.repositories.scheme_seats import (
     bulk_remap_scheme_version_seats,
     list_scheme_version_seats,
 )
+from app.services.api_errors import raise_unprocessable
 from app.services.editor_validation import validate_remap_target_references
 
 
@@ -35,9 +34,9 @@ async def preview_remap(
     to_group_id: str | None,
 ) -> list[dict]:
     if not any([seat_record_ids, from_sector_id, from_group_id]):
-        raise HTTPException(
-            status_code=status.HTTP_422_UNPROCESSABLE_ENTITY,
-            detail="At least one remap filter must be provided",
+        raise_unprocessable(
+            code="remap_filter_required",
+            message="At least one remap filter must be provided",
         )
 
     await validate_remap_target_references(