v2.0: feat: no double-booking — switch between events at the same time

- Registering, starting a team or accepting a team invite is refused when it overlaps an event the
  student is registered or waitlisted for (SCHEDULE_CONFLICT with the clashing events)
- Switch: the student agrees to give up the overlapping places (only before those events start);
  they're cancelled through the normal cancel path (seats go to the waitlist, leaders withdraw their
  team) and the new registration goes ahead
- Event page warns in advance and offers "Switch to this event"; one dialog everywhere explains what
  switching gives up; team invite picker shows who's busy at that time
- Tests: scheduleClash (backend), ScheduleClash (frontend); test setup waits up to 5s so the parallel
  suite isn't flaky on a busy machine

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>