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

apps/backend/src/app.ts

@@ -0,0 +1,49 @@
+import { Hono } from 'hono';
+import type { BackendEnv } from '@/http/env';
+import {
+  internalServerErrorProblem,
+  notFoundProblem,
+  problemJson,
+} from '@/http/problem-details';
+import { requestIdMiddleware } from '@/http/request-id';
+import { requestLoggerMiddleware } from '@/http/request-logger';
+import { corsMiddleware, securityHeadersMiddleware } from '@/http/security-headers';
+import { sessionAuthMiddleware } from '@/http/session-auth';
+import { logger } from '@/logger';
+import { authRoutes } from './modules/auth';
+import { groupsRoutes } from './modules/groups';
+import { healthCheckRoutes } from './modules/health-check';
+import { paymentsRoutes } from './modules/payments';
+import { studentsRoutes } from './modules/students';
+import { waitlistRoutes } from './modules/waitlist';
+
+const app = new Hono<BackendEnv>();
+
+app.use('*', corsMiddleware);
+app.use('*', securityHeadersMiddleware);
+app.use('*', requestIdMiddleware);
+app.use('*', requestLoggerMiddleware);
+
+app.get('/', (c) => c.json({ name: 'Gruperly API', status: 'ok' }));
+
+const api = app.basePath('/api/v1');
+
+api.use('*', sessionAuthMiddleware);
+
+api.route('/auth', authRoutes);
+api.route('/health', healthCheckRoutes);
+api.route('/groups', groupsRoutes);
+api.route('/students', studentsRoutes);
+api.route('/payments', paymentsRoutes);
+api.route('/waitlist', waitlistRoutes);
+
+app.notFound((c) => {
+  return problemJson(c, notFoundProblem(c.req.path));
+});
+
+app.onError((error, c) => {
+  logger.error({ error, path: c.req.path }, 'Unhandled request error');
+  return problemJson(c, internalServerErrorProblem(c.req.path));
+});
+
+export default app;