feat(backend): add stale draft guards and reference validation for draft mutations

add stale draft protection for mutation flows

validate referenced entities before applying draft changes
reduce invalid draft writes caused by stale state and broken references

keep mutation behavior explicit and version-aware

backend/app/services/remap_service.py

@@ -6,6 +6,7 @@ from app.repositories.scheme_seats import (
     bulk_remap_scheme_version_seats,
     list_scheme_version_seats,
 )
+from app.services.editor_validation import validate_remap_target_references
 
 
 def _match_seat(
@@ -39,6 +40,12 @@ async def preview_remap(
             detail="At least one remap filter must be provided",
         )
 
+    await validate_remap_target_references(
+        scheme_version_id=scheme_version_id,
+        to_sector_id=to_sector_id,
+        to_group_id=to_group_id,
+    )
+
     seats = await list_scheme_version_seats(scheme_version_id)
     seat_record_id_set = set(seat_record_ids) if seat_record_ids else None