Events: Cancellation + Waitlist promotion

[danielhe4rt]

Parent

#237

What to build

Implement cancellation with deadline validation and automatic waitlist promotion (FIFO) when a confirmed participant cancels.

Action: CancelEnrollmentAction

• Validates: enrollment status must be confirmed, waitlisted, or pending
• Validates deadline: now() < event.starts_at - policy.cancellation_deadline_hours
• Transitions enrollment to cancelled, sets cancelled_at timestamp
• Records transition in audit trail (triggered_by=user, reason optional)
• If the cancelled enrollment was confirmed AND there are waitlisted enrollments: trigger promotion

Action: PromoteFromWaitlistAction

• Finds the oldest waitlisted enrollment (ordered by enrolled_at ASC, LIMIT 1)
• Transitions it to confirmed, sets confirmed_at, clears waitlist_position
• Records transition (triggered_by=system, reason="slot opened from cancellation")
• Dispatches EnrollmentConfirmed domain event for the promoted enrollment
• Recalculates waitlist_position for remaining waitlisted enrollments (optional optimization: just leave gaps)

App panel:

• "Cancel" button visible on enrollment when:
  • Status is confirmed, waitlisted, or pending
  • Deadline has not passed
• Shows "Cancellation deadline passed" message when deadline expired
• Confirmation dialog before cancelling

Admin panel:

• "Cancel Enrollment" action available on enrollment records (bypasses deadline — admin override)
• Shows cancellation reason in transitions log

Edge cases:

• Cancellation after deadline: rejected with 422 and message showing deadline
• Cancellation of waitlisted: does NOT trigger promotion (no slot was freed)
• Multiple cancellations in quick succession: each promotes the next in line

Acceptance criteria

• Participant can cancel within deadline → status changes to cancelled
• Cancellation after deadline is rejected with clear error message
• Cancelling a confirmed enrollment promotes next waitlisted participant (FIFO)
• Promoted participant gets confirmed status with proper transition record
• EnrollmentConfirmed is dispatched for promoted enrollment
• Cancelling a waitlisted enrollment does not trigger promotion
• Admin can cancel any enrollment regardless of deadline
• Transition audit trail records actor and reason for all cancellations
• Feature tests: cancel within deadline, cancel after deadline, waitlist promotion, admin override
• Pint passes

Blocked by

• Events: Waitlist + Capacity enforcement #241 (Waitlist + Capacity)