[NathanLe2247]

What

Users could still submit or change an RSVP after an event had started, including from the Past section. RSVPs should close at the event's start time.

Fix

• Server: rsvpToEvent now rejects any RSVP (new or changed) once event.startTime has passed, with a 400 and the message "RSVPs are closed because this event has already started." (EventController.ts)
• Attendance page (EventAttendance.tsx):
  • No RSVP yet and the event has started: shows "RSVPs are closed" instead of the Accept/Decline form.
  • Already RSVP'd: keeps the Accepted/Declined badge and replaces "Update RSVP" with "RSVPs closed when this event started."
  • The error toast now shows the server's message instead of only the HTTP status text.

Verified

• ./nx build pplserver and ./nx build pplui pass
• Upcoming event: can RSVP and change it
• Started/past event: form and Update button are replaced with the closed message