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/.env.example

@@ -0,0 +1,21 @@
+# PostgreSQL connection string
+DATABASE_URL="postgresql://postgres:postgres@localhost:5432/gruperly?schema=public"
+
+# Frontend origin (CORS + CSRF)
+WEB_URL="http://localhost:6173"
+
+# Better Auth
+BETTER_AUTH_SECRET="replace-with-a-strong-secret"
+BETTER_AUTH_URL="http://localhost:4000"
+
+# Social providers
+GOOGLE_CLIENT_ID=""
+GOOGLE_CLIENT_SECRET=""
+
+# Email (SMTP por ahora; migrar a un proveedor transaccional en el futuro)
+EMAIL_PROVIDER="smtp"
+EMAIL_SERVER_HOST=""
+EMAIL_SERVER_PORT="587"
+EMAIL_SERVER_USER=""
+EMAIL_SERVER_PASSWORD=""
+EMAIL_FROM="Gruperly <no-reply@gruperly.com>"