Refactor app helpers and use AppStateDep in config router

Move service-dependent helper wrappers from app.utils to app.helpers and update config router activation/rollback to use explicit AppState dependency.

backend/app/helpers/__init__.py

@@ -0,0 +1,6 @@
+"""Cross-service helpers and shared abstractions.
+
+Modules in ``app.helpers`` are allowed to depend on the service layer when they
+implement shared business logic used by multiple service modules. This keeps
+``app.utils`` independent and low-level.
+"""