[Pipeline B] Route Controller/FileManipulator through the form service

[abhishek-8081]

The form-fill logic instantiates Controller() directly in the routes and in the Celery task
(app/tasks/fill.py). With a FormService in place (see the layering issue), the
Controller/FileManipulator invocation should go through the service rather than being
constructed ad-hoc in the route/task.

Changes

• FormService owns the call into Controller/FileManipulator for filling.
• routes/forms.py and the fill Celery task call FormService instead of instantiating
  Controller() directly.

Acceptance criteria

• Controller/FileManipulator is invoked via FormService, not constructed directly in routes
  or tasks.
• Sync and async fill paths both go through the service.
• Behavior unchanged; tests pass.

Part of the Pipeline B tracker (#636 ). Depends on the layering issue.