[Pipeline B] Route Controller/FileManipulator through FormService (#641)

[abhishek-8081]

Closes #641.

The async fill task (app/tasks/fill.py) was constructing Controller() directly and building
its own FormSubmission. It now routes through FormService like the sync path does (after
#640), so Controller is constructed in exactly one place (FormService.init).

• New FormService.fill_and_persist(session, template, transcript, input_id, model): calls
  Controller and persists the FormSubmission. Both the sync route and the async task use it.
• FormService.fill_form (sync) now resolves the transcript, then delegates to
  fill_and_persist.
• fill_form_task drops its direct Controller() and inline FormSubmission build, and calls
  fill_and_persist with the already-resolved transcript (jobs.py resolves up front per [Pipeline B] Wire the fill flow to accept input_id and use the stored transcript #638
  — no re-resolution). Job-status handling stays in the task.
• weather.py / zipcode.py use Controller for their own purposes and are intentionally
  untouched — [Pipeline B] Route Controller/FileManipulator through the form service #641 is only the form-fill path.

Behavior unchanged — 152 tests pass (no test changes needed; the task is mocked at the
dispatch boundary). ruff check app/ clean. No Controller() construction remains in
app/tasks/fill.py.