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/test/invitations.test.ts

@@ -8,6 +8,11 @@ const db = {
   attendee: {
     findFirst: mock(),
     create: mock(),
+    count: mock(),
+  },
+  groupWaitlistEntry: {
+    findFirst: mock(),
+    create: mock(),
   },
 };
 
@@ -151,5 +156,48 @@ describe('public invitations routes', () => {
 
       expect(res.status).toBe(400);
     });
+
+    it('adds to the waitlist when the group is full', async () => {
+      prisma.group.findUnique.mockResolvedValue(mockGroup as never);
+      prisma.attendee.findFirst.mockResolvedValue(null);
+      prisma.attendee.count.mockResolvedValue(15); // capacity full
+      prisma.groupWaitlistEntry.findFirst.mockResolvedValue(null);
+      prisma.groupWaitlistEntry.create.mockResolvedValue({
+        id: 'wl-1',
+        groupId: 'group-1',
+        fullName: 'Lucía Méndez',
+        phone: '+5491122334455',
+        email: 'lucia@test.com',
+        notes: null,
+        status: 'PENDING',
+        createdAt: new Date('2026-09-18T10:00:00.000Z'),
+        updatedAt: new Date('2026-09-18T10:00:00.000Z'),
+      } as never);
+
+      const res = await app.request('/invitations/valid-token-123/join', {
+        method: 'POST',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify({
+          firstName: 'Lucía',
+          lastName: 'Méndez',
+          phone: '+54 9 11 2233-4455',
+          email: 'lucia@test.com',
+        }),
+      });
+
+      expect(res.status).toBe(201);
+      const body = await res.json();
+      expect(body.status).toBe('waitlisted');
+      expect(body.attendee).toBeNull();
+      expect(prisma.attendee.create).not.toHaveBeenCalled();
+      expect(prisma.groupWaitlistEntry.create).toHaveBeenCalledWith({
+        data: {
+          groupId: 'group-1',
+          fullName: 'Lucía Méndez',
+          phone: '+5491122334455',
+          email: 'lucia@test.com',
+        },
+      });
+    });
   });
 });