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/modules/group-waitlist/lib/helpers.ts

@@ -0,0 +1,65 @@
+import type { Group, PrismaClient } from '@generated/prisma/client';
+import type { GroupWaitlistEntryDto, ProblemDetails, Result } from '@gruperly/shared';
+import { err, ok } from '@gruperly/shared';
+import { noGroupAccessProblem, notFoundResourceProblem } from '@/http/problem-builders';
+
+export type GroupWaitlistEntryRecord = {
+  id: string;
+  groupId: string;
+  fullName: string;
+  phone: string;
+  email: string | null;
+  notes: string | null;
+  status: GroupWaitlistEntryDto['status'];
+  createdAt: Date;
+  updatedAt: Date;
+};
+
+export function toGroupWaitlistEntryDto(record: GroupWaitlistEntryRecord): GroupWaitlistEntryDto {
+  return {
+    id: record.id,
+    groupId: record.groupId,
+    fullName: record.fullName,
+    phone: record.phone,
+    email: record.email,
+    notes: record.notes,
+    status: record.status,
+    createdAt: record.createdAt.toISOString(),
+    updatedAt: record.updatedAt.toISOString(),
+  };
+}
+
+export async function findGroupForUser(
+  db: Pick<PrismaClient, 'group'>,
+  groupId: string,
+  userId: string,
+): Promise<Result<Group, ProblemDetails>> {
+  const group = await db.group.findUnique({
+    where: { id: groupId },
+    include: { members: { where: { userId } } },
+  });
+  if (!group) {
+    return err(notFoundResourceProblem('Group', groupId));
+  }
+  const isOwner = group.createdById === userId;
+  const isMember = group.members.length > 0;
+  if (!isOwner && !isMember) {
+    return err(noGroupAccessProblem());
+  }
+  return ok(group);
+}
+
+export async function findGroupOwnedByUser(
+  db: Pick<PrismaClient, 'group'>,
+  groupId: string,
+  userId: string,
+): Promise<Result<Group, ProblemDetails>> {
+  const group = await db.group.findUnique({ where: { id: groupId } });
+  if (!group) {
+    return err(notFoundResourceProblem('Group', groupId));
+  }
+  if (group.createdById !== userId) {
+    return err(noGroupAccessProblem());
+  }
+  return ok(group);
+}