feat: add group waitlist functionality

- Introduced GroupWaitlistEntry model in Prisma schema to manage waitlisted users for groups.
- Implemented API route to add users to the group waitlist with validation.
- Enhanced attendee creation logic to handle group capacity and waitlisting scenarios.
- Added new problem builders for handling waitlist-related errors.
- Updated frontend to support waitlist interactions, including modals for capacity warnings.
- Created tests for waitlist functionality, ensuring proper handling of full groups and existing waitlist entries.

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

@@ -100,4 +100,20 @@ export function onboardingAlreadyCompletedProblem(): ProblemDetails {
     detail: 'Ya completaste el onboarding de Gruperly.',
     code: 'onboarding_already_completed',
   });
+}
+
+export function capacityReachedProblem(capacity: number | null): ProblemDetails {
+  return conflictProblem({
+    detail: capacity
+      ? `El grupo alcanzó su cupo máximo de ${capacity} miembros.`
+      : 'El grupo alcanzó su cupo máximo de miembros.',
+    code: 'group_capacity_reached',
+  });
+}
+
+export function alreadyWaitlistedProblem(): ProblemDetails {
+  return conflictProblem({
+    detail: 'Este número de teléfono ya está en la lista de espera del grupo.',
+    code: 'already_waitlisted',
+  });
 }