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/prisma/models/domain.prisma

@@ -19,6 +19,7 @@ model Group {
   members   GroupMember[]
   attendees Attendee[]
   payments  Payment[]
+  waitlist  GroupWaitlistEntry[]
 
   @@map("groups")
 }
@@ -120,6 +121,24 @@ model WaitlistEntry {
   @@map("waitlist_entries")
 }
 
+model GroupWaitlistEntry {
+  id        String         @id @default(cuid())
+  groupId   String
+  fullName  String
+  phone     String
+  email     String?
+  notes     String?
+  status    WaitlistStatus @default(PENDING) // PENDING, INVITED, JOINED, DECLINED
+  createdAt DateTime       @default(now())
+  updatedAt DateTime       @updatedAt
+
+  group Group @relation(fields: [groupId], references: [id], onDelete: Cascade)
+
+  @@unique([groupId, phone])
+  @@index([groupId])
+  @@map("group_waitlist_entries")
+}
+
 enum WaitlistStatus {
   PENDING
   INVITED