EventDetail missing capacity_remaining — inconsistent with EventListItem

## Bug EventListItem (from event list endpoints, #4167) returns capacity_remaining: int | None computed server-side. But EventDetail (from event detail endpoint, #4166) does not include this field. This forces the frontend to compute capacity - total_guests itself for the detail page, while the list view gets it pre-computed. Same business logic in two places = inconsistent contract. ## Expected Both EventDetail and EventListItem should return capacity_remaining, computed the same way server-side: capacity - total_guests (null when no capacity set). ## Fix Add capacity_remaining: int | None to EventDetail response, computed as capacity - rsvp_counts.total_guests. ## Found by API contract audit (2026-03-19)