[Enhancement] Events RSVP + calendar polish (iCal, capacity, waitlist, recurring)

What: Events exist but light. Add: iCal export per event, member RSVP with yes/no/maybe, attendee list, capacity, waitlist, reminder emails (we have email infra), recurring events, time-zone-aware display.

Who benefits: Creators running live events / AMAs; communities with regular cadence; member onboarding.

Why: Mighty's killer feature. We already have events as a surface; this is the round of polish that turns "we have events" into "events that members actually use."

Dependencies: Email queue infra (shipped). Push notifications (shipped).

Recommended order: Sprint 2 (week 3-4). Can pair with E (live chat) when an event uses both.

Complexity: 1 week. Backend mostly there; the work is RSVP model + UI.

Priority: high