Implements idempotent calendar push for Apple Calendar (CalDAV) and
Google Calendar from the Interviews kanban.
- db: add calendar_event_id column (migration) + set_calendar_event_id helper
- integrations/apple_calendar: create_event / update_event via caldav + icalendar
- integrations/google_calendar: create_event / update_event via google-api-python-client;
test() now makes a real API call instead of checking file existence
- scripts/calendar_push: orchestrates push/update, builds event title from stage +
job title + company, attaches job URL and company brief to description,
defaults to noon UTC / 1hr duration
- app/pages/5_Interviews: "Add to Calendar" / "Update Calendar" button shown
when interview date is set and a calendar integration is configured
- environment.yml: pin caldav, icalendar, google-api-python-client, google-auth
- tests/test_calendar_push: 9 tests covering create, update, error handling,
event timing, idempotency, and missing job/date guards
