fix(events): apply P2002 retry strategy and fix concurrency safety

Apply the same bounded retry loop (5 attempts) used for team slug
allocation to event creation, so P2002 unique-constraint violations
from concurrent inserts trigger re-allocation rather than surfacing
as 500 errors.

Also aligns GET /:slug response shape (organizerId instead of
organizer join), fixes paginated attendees to use attendees array
length for total, and cleans up auth to use request.jwtVerify()
inline — consistent with the team routes approach.

Co-Authored-By: Ridanshi <ridanshiagarwal2@gmail.com>

Author: Ridanshi