feat(events): waitlist and capacity enforcement

[danielhe4rt]

Parent

#237

What to build

Add atomic capacity checking to the enrollment flow. When an event is full, participants are placed on a waitlist (FIFO) if the policy allows, otherwise enrollment is rejected.

Modify EnrollUserAction:

• Inside DB::transaction, use lockForUpdate() on the enrollment policy to prevent race conditions
• Count current confirmed + checked_in + attended enrollments for the event
• If count < capacity (or capacity is null/unlimited): confirm as before
• If count >= capacity AND has_waitlist = true: create enrollment with status waitlisted, assign waitlist_position (max position + 1)
• If count >= capacity AND has_waitlist = false: throw validation exception (422 "Event is full")
• Record appropriate transition in audit trail

Domain event:

• Dispatch EnrollmentWaitlisted (for future notifications) implementing ShouldDispatchAfterCommit

Admin panel:

• Waitlist column visible in enrollments RelationManager
• Filter enrollments by status (confirmed, waitlisted, etc.)

App panel:

• Show "You are on the waitlist (position X)" when status is waitlisted
• Show "Event is full" message when capacity reached and no waitlist

Scopes on EventEnrollment model:

• scopeConfirmed(), scopeWaitlisted(), scopeActive() (confirmed + checked_in + attended)

Acceptance criteria

• Enrollment respects capacity limit — no overbooking under concurrent requests
• Waitlisted enrollment gets correct waitlist_position (FIFO)
• Event without waitlist returns 422 when full
• Event with unlimited capacity (null) always confirms
• lockForUpdate() is used inside transaction for atomicity
• Admin panel shows waitlist status and position
• App panel shows waitlist position to participant
• Feature tests: capacity enforcement, waitlist assignment, concurrent enrollment simulation, unlimited capacity
• Pint passes

Blocked by

• feat(events): RSVP enrollment end-to-end #240 (RSVP Enrollment)