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/migrations/20260922191756_add_group_waitlist/migration.sql

@@ -0,0 +1,23 @@
+-- CreateTable
+CREATE TABLE "group_waitlist_entries" (
+    "id" TEXT NOT NULL,
+    "groupId" TEXT NOT NULL,
+    "fullName" TEXT NOT NULL,
+    "phone" TEXT NOT NULL,
+    "email" TEXT,
+    "notes" TEXT,
+    "status" "WaitlistStatus" NOT NULL DEFAULT 'PENDING',
+    "createdAt" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    "updatedAt" TIMESTAMP(3) NOT NULL,
+
+    CONSTRAINT "group_waitlist_entries_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateIndex
+CREATE INDEX "group_waitlist_entries_groupId_idx" ON "group_waitlist_entries"("groupId");
+
+-- CreateIndex
+CREATE UNIQUE INDEX "group_waitlist_entries_groupId_phone_key" ON "group_waitlist_entries"("groupId", "phone");
+
+-- AddForeignKey
+ALTER TABLE "group_waitlist_entries" ADD CONSTRAINT "group_waitlist_entries_groupId_fkey" FOREIGN KEY ("groupId") REFERENCES "groups"("id") ON DELETE CASCADE ON UPDATE CASCADE;