Prepare rollback-safe legacy migration data contract (#7)

apps/core/middleware.py

@@ -9,6 +9,30 @@ from django.shortcuts import redirect
 from django.urls import reverse
 from django.utils.http import url_has_allowed_host_and_scheme
 
+from scripts.migration_worker_gate import hold_active
+
+
+class MigrationMaintenanceMiddleware:
+    """Block candidate traffic before durable migration activation, including GET writes."""
+
+    def __init__(self, get_response: Callable[[HttpRequest], HttpResponse]) -> None:
+        self.get_response = get_response
+
+    def __call__(self, request: HttpRequest) -> HttpResponse:
+        try:
+            held = hold_active()
+        except (OSError, RuntimeError):
+            held = True
+        health = request.method in {"GET", "HEAD"} and request.path_info in {
+            "/health/ready/", "/health/live/",
+        }
+        if held and not health:
+            response = JsonResponse({"error": "migration_maintenance"}, status=503)
+            response["Retry-After"] = "30"
+            response["Cache-Control"] = "no-store"
+            return response
+        return self.get_response(request)
+
 
 class DemoReadOnlyMiddleware:
     """Prevent a shared public demo account from mutating application data."""