Events: CRUD + RSVP + calendar export

[MrImmortal09]

Context

Move events off mock data and let admins create / edit events. Let users RSVP.

Acceptance criteria

• Event model with fields matching lib/data.ts
• Admin UI /admin/events for create / edit / delete
• Rich-text description (MDX or tiptap)
• Event detail page (gated behind auth) with RSVP button
• RSVP model: { eventId, userId, status: YES | NO | MAYBE }
• Attendee list visible on the detail page — respects per-user visibility settings (hide RSVP from list if user opted out)
• .ics calendar export endpoint
• Email reminder 24 h before the event to RSVPed users

Depends on #1, #7, #8, #22.