feat(backend): add draft validation endpoint with stale version guard

add backend endpoint for draft validation

protect validation requests against stale version state
keep draft checks version-aware and consistent with mutation flows

backend/app/api/routes/editor.py

@@ -56,6 +56,7 @@ from app.services.editor_validation import (
     validate_single_seat_patch_references,
     validate_single_seat_patch_uniqueness,
 )
+from app.services.scheme_validation import build_scheme_validation_report
 from app.services.structure_diff import build_structure_diff
 from app.services.structure_sync import repair_structure_references
 
@@ -131,6 +132,28 @@ async def get_draft_structure(
     )
 
 
+@router.get(f"{settings.api_v1_prefix}/schemes/{{scheme_id}}/draft/validation")
+async def get_draft_validation(
+    scheme_id: str,
+    expected_scheme_version_id: str | None = Query(default=None),
+    role: str = Depends(require_api_key),
+):
+    scheme, version = await get_current_draft_context(
+        scheme_id,
+        expected_scheme_version_id=expected_scheme_version_id,
+    )
+    report = await build_scheme_validation_report(
+        scheme_id=scheme.scheme_id,
+        scheme_version_id=version.scheme_version_id,
+    )
+    return {
+        "scheme_id": scheme.scheme_id,
+        "scheme_version_id": version.scheme_version_id,
+        "status": version.status,
+        "report": report,
+    }
+
+
 @router.get(f"{settings.api_v1_prefix}/schemes/{{scheme_id}}/draft/compare-preview", response_model=StructureDiffResponse)
 async def get_draft_compare_preview(
     scheme_id: str,