refactor: migrate backend to pnpm monorepo with Hono module architecture

- Replace Bun with pnpm 9 + Turborepo + tsx; apps/api renamed to apps/backend
- Split backend into http/, modules/, lib/ layers mirroring tai-specguard
- Add use-case + Result pattern, Problem Details RFC 7807, basePath /api/v1
- Mount Better Auth at /api/v1/auth, keep session-auth whitelist
- Split Prisma schema into prisma/models/*, generate into generated/
- Rework packages/shared into lib/ + schemas/ with pagination DTOs
- Implement health, groups, students, payments, waitlist modules
- Add vitest suite with prisma mocks (21 tests), biome lint
- Point web client to /api/v1/auth and /api/v1/groups/from-organization

packages/shared/src/schemas/waitlist.ts

@@ -0,0 +1,31 @@
+import { z } from 'zod';
+import { createPageSchema, createPageSizeSchema, createPaginationSchema } from './pagination.js';
+
+const isoDateTimeSchema = z.string().datetime();
+const pageSchema = createPageSchema();
+const pageSizeSchema = createPageSizeSchema(10);
+const paginationSchema = createPaginationSchema();
+
+export const waitlistStatusSchema = z.enum(['PENDING', 'INVITED', 'JOINED', 'DECLINED']);
+
+export const WaitlistEntryDtoSchema = z.object({
+  id: z.string(),
+  email: z.string(),
+  name: z.string().nullable(),
+  status: waitlistStatusSchema,
+  createdAt: isoDateTimeSchema,
+  updatedAt: isoDateTimeSchema,
+});
+export type WaitlistEntryDto = z.output<typeof WaitlistEntryDtoSchema>;
+
+export const WaitlistQuerySchema = z.object({
+  page: pageSchema,
+  pageSize: pageSizeSchema,
+});
+export type WaitlistQuery = z.output<typeof WaitlistQuerySchema>;
+
+export const WaitlistListSchema = z.object({
+  data: z.array(WaitlistEntryDtoSchema),
+  pagination: paginationSchema,
+});
+export type WaitlistList = z.output<typeof WaitlistListSchema>;