v1.2.0

feat: pymissive and django-pymissive 1.2.0 — scheduled campaign runners

- Add missive_type filter, task_object GFK, and external_task_backend on
  MissiveScheduledCampaign with admin UI (start/reschedule/duplicate actions)
- Unify execution through run_with_tracking → run_campaign with atomic claims,
  best-effort process_missives (ERROR on failure), and backend allowlist validation
- Add fakeapp hook/runner fixtures for testing custom task backends
- Make campaign description optional in model and admin
- Add test_scheduled_campaign.py covering runners, tracking, and validation

Co-authored-by: Cursor <cursoragent@cursor.com>