fix: stop orphaning google calendar events on schedule re-upload

[jaspermayone]

Closes #500

Every course event was appearing twice (or more) in users' Google Calendars.

Root cause

Since 035a4bd (June 13), CourseProcessorService ran course.meeting_times.destroy_all on every POST /api/process_courses (every extension schedule upload). Meeting times carried has_many :google_calendar_events, dependent: :destroy, so the tracking rows were destroyed without any Google API call — the real events stayed on the calendar, now invisible to the app. The re-ingested meeting times got new IDs, so the next sync created a fresh copy of every event next to the stranded one. Each re-upload added another generation, for every user enrolled in the course, and no cleanup job could see the orphans because nothing reconciles against the Google side.

sequenceDiagram
    participant E as Extension
    participant P as CourseProcessorService
    participant DB as google_calendar_events
    participant G as Google Calendar

    E->>P: POST /api/process_courses
    P->>DB: meeting_times.destroy_all (cascade)
    Note over DB,G: rows destroyed, no API call —<br/>events stranded in Google
    P->>P: re-ingest meeting times (new IDs)
    P->>G: next sync: no tracked event for new IDs
    G->>G: duplicate created next to orphan

Loading

Changes

• CourseProcessorService: upsert meeting times in place instead of destroy-all-and-recreate. MeetingTimesIngestService now returns the IDs it touched; only rows absent from the upload are destroyed. This keeps meeting time IDs (and therefore event tracking rows and event_preference records, which were also being wiped every upload) stable.
• Course::MeetingTime / FinalExam: google_calendar_events association changed from dependent: :destroy to dependent: :nullify. The tracking row is the only pointer to the real Google event — orphaned rows are now picked up by CleanupOrphanedCalendarEventsJob, which deletes from Google before removing the row. (UniversityCalendarEvent already worked this way.)
• CleanupUntrackedGoogleEventsJob (new, one-time remediation): lists events on each app-managed calendar via the service account and deletes any not tracked in google_calendar_events. Skips cancelled events and user-edited instances of tracked recurring series. Supports dry_run: true and per-calendar scoping.

Remediation rollout

CleanupUntrackedGoogleEventsJob.perform_now(nil, dry_run: true)  # inspect logs first
CleanupUntrackedGoogleEventsJob.perform_later                    # then delete for real

Tests

• CourseProcessorService: meeting time IDs stable across re-processing, tracking rows preserved, stale meeting times removed with events nullified
• MeetingTimesIngestService: returns touched IDs, reuses rows on re-ingest
• GoogleCalendarEvent: nullified (not destroyed) when meeting time / final exam is destroyed
• CleanupUntrackedGoogleEventsJob: deletes only untracked events, dry-run mode, 404 tolerance

Full suite passes locally (bundle exec rspec, 19 examples, 0 failures); rubocop clean.