Add deregistration logic

[steeevin88]

CONTEXT

• users can register for events and join waitlists, but there's no way to cancel or deregister
• no DELETE /events/:id/register endpoint exists
• this means a user who can no longer attend is permanently holding a seat, and waitlisted users have no chance of getting in unless an admin manually intervenes

CHANGES NEEDED

• add a deregister/cancel endpoint (e.g. DELETE /events/:id/register)
• release the held seat in Redis for capacity-limited events
• mark the registration document as canceled (or a new terminal status) rather than deleting it — keep the audit trail
• if a waitlist exists and is enabled for the event, trigger promotion of the next waitlisted user
• restrict to the registrant themselves or event admins

APPROACH

• deregistration flow triggers ReleaseEventSeat in Redis, then promotes from waitlist via the Kafka pipeline (same as auto-promotion).