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

stack.md

@@ -7,11 +7,12 @@ Este documento define la arquitectura, convenciones y especificaciones del stack
 ## 1. Stack Tecnológico Principal
 
 ### Runtime & Monorepo Manager
-* **Runtime / Package Manager:** [Bun](https://bun.sh/) (última versión estable)
-* **Monorepo Tooling:** Bun Workspaces (opcionalmente orquestado con Turborepo)
+* **Runtime:** Node.js (>=20)
+* **Package Manager:** [pnpm](https://pnpm.io/) 9
+* **Monorepo Tooling:** pnpm workspaces + Turborepo
 
-### Backend (`apps/api`)
-* **Framework:** [Hono](https://hono.dev/) (para Bun)
+### Backend (`apps/backend`)
+* **Framework:** [Hono](https://hono.dev/) (servido con `@hono/node-server`, ejecutado con `tsx`)
 * **ORM:** [Prisma](https://www.prisma.io/)
 * **Base de Datos:** PostgreSQL
 * **Autenticación:** [Better Auth](https://www.better-auth.com/)
@@ -36,21 +37,24 @@ Este documento define la arquitectura, convenciones y especificaciones del stack
 ```text
 gruperly/
 ├── apps/
-│   ├── api/                     # Servicio Backend (Bun + Hono)
+│   ├── backend/                  # Servicio Backend (Node + Hono + tsx)
 │   │   ├── src/
-│   │   │   ├── db/              # Cliente e inicialización de Prisma
-│   │   │   ├── lib/             # Instancia de Better Auth y helpers
-│   │   │   ├── routes/          # Endpoints modularizados de Hono
-│   │   │   │   ├── auth.ts
-│   │   │   │   ├── groups.ts
-│   │   │   │   ├── students.ts
-│   │   │   │   ├── payments.ts
-│   │   │   │   └── waitlist.ts
-│   │   │   └── index.ts         # Entry point de la API
+│   │   │   ├── http/             # Infraestructura HTTP (validate, problem-details, session-auth, ...)
+│   │   │   ├── lib/              # Prisma (cliente + UnitOfWork), pagination, email, helpers
+│   │   │   ├── modules/          # Módulos: health-check, auth, groups, students, payments, waitlist
+│   │   │   │   └── <módulo>/
+│   │   │   │       ├── routes.ts
+│   │   │   │       └── features/<accion>/{route,use-case}.ts
+│   │   │   ├── app.ts            # Monta basePath('/api/v1') + rutas + notFound/onError (RFC 7807)
+│   │   │   ├── server.ts         # Entry point (serve de @hono/node-server)
+│   │   │   └── logger.ts         # pino + pino-pretty
 │   │   ├── prisma/
-│   │   │   ├── schema.prisma    # Definición del modelo de datos
-│   │   │   └── generated/       # Cliente generado de Prisma (gitignoreado)
-│   │   ├── prisma.config.ts     # Config de Prisma CLI (datasource URL, migrations)
+│   │   │   ├── schema.prisma     # Generator + datasource simple
+│   │   │   ├── models/           # Modelos multi-archivo (auth.prisma, domain.prisma)
+│   │   │   └── migrations/
+│   │   ├── generated/            # Cliente generado de Prisma (gitignoreado, alias @generated/*)
+│   │   ├── prisma.config.ts      # Config de Prisma CLI (datasource URL, migrations)
+│   │   ├── test/                 # Tests vitest
 │   │   ├── tsconfig.json
 │   │   └── package.json
 │   │