feat(ui): add RSVP attendance option and admin features

* Implement attendance selection in RSVP form
* Add admin endpoints for managing RSVPs and guests
* Update RSVP handling to include attendance status

server/internal/webhook/webhook.go

@@ -78,8 +78,9 @@ func (c *Client) SendRSVP(r store.RSVP) error {
 		guests[i] = map[string]any{"name": g.Name, "isChild": g.IsChild}
 	}
 	return c.post(c.cfg.RSVPURL, "rsvp.created", map[string]any{
-		"guests":  guests,
-		"message": r.Message,
+		"guests":    guests,
+		"message":   r.Message,
+		"attending": r.Attending,
 	})
 }