Link FormSubmission to Input by foreign key (#639)

[abhishek-8081]

Closes #639.

Adds a nullable input_id foreign key on FormSubmission referencing Input, so a submission
links to the Input it was filled from instead of only duplicating the transcript.

• Model: FormSubmission.input_id (UUID FK to inputs.input_id, nullable), matching the
  existing Extraction.input_id style. input_text kept.
• Migration 003 (down_revision 002): adds the FK via batch_alter_table so it works on both
  SQLite (tests) and Postgres — a plain ALTER-add-constraint isn't supported on SQLite.
  Round-trips cleanly (upgrade → downgrade → re-upgrade).
• Sync fill (forms.py) sets input_id directly; async fill threads input_id into
  fill_form_task (as a string, per the Celery serialization convention already used in
  transcribe.py) and jobs.py passes it in.
• input_text is intentionally KEPT — the /forms/submissions and analytics endpoints still
  read it. Migrating those readers to the FK and dropping input_text is a deferred
  follow-up (noted in the migration docstring).

@marcvergees @vharkins1 @chetanr25