feat: manage waitlist and remove group members

- Add group-waitlist module (list/promote/remove entries)
- Remove attendee with optional atomic promotion from waitlist
- Block removal when attendee has payments (attendee_has_payments)
- Waitlist detail modal matching members UI; optimistic add-to-waitlist
- Responsive tweaks for the members/waitlist tabs

apps/backend/src/http/problem-builders.ts

@@ -116,4 +116,11 @@ export function alreadyWaitlistedProblem(): ProblemDetails {
     detail: 'Este número de teléfono ya está en la lista de espera del grupo.',
     code: 'already_waitlisted',
   });
+}
+
+export function attendeeHasPaymentsProblem(): ProblemDetails {
+  return conflictProblem({
+    detail: 'Este miembro tiene cobros asociados. Gestiona o cancela sus cobros antes de quitarlo del grupo.',
+    code: 'attendee_has_payments',
+  });
 }