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

README.md

@@ -4,17 +4,17 @@ Monorepo de Gruperly. Consulta [stack.md](./stack.md) para arquitectura y especi
 
 ## Stack
 
-- Runtime & package manager: [Bun](https://bun.sh/)
-- Monorepo: Bun Workspaces + Turborepo
-- Backend (`apps/api`): Bun + Hono + Prisma + PostgreSQL + Better Auth
+- Package manager: [pnpm](https://pnpm.io/) 9
+- Monorepo: pnpm workspaces + Turborepo
+- Backend (`apps/backend`): Node + Hono (`@hono/node-server`) + Prisma + PostgreSQL + Better Auth
 - Frontend (`apps/web`): React 19 + TypeScript + TanStack Router + TanStack Query + Tailwind v4
-- Shared (`packages/shared`): esquemas Zod y tipos compartidos
+- Shared (`packages/shared`): esquemas Zod, tipos, `Result` y Problem Details compartidos
 
 ## Estructura
 
 ```text
 apps/
-  api/      # Backend (Bun + Hono + Prisma)
+  backend/  # Backend (Node + Hono + Prisma, tsx)
   web/      # Frontend (React PWA)
 packages/
   shared/   # Esquemas Zod y tipos compartidos
@@ -24,13 +24,16 @@ packages/
 ## Comandos
 
 ```bash
-bun install          # Instala dependencias de todos los workspaces
-bun run dev          # Levanta la API y la web (vía Turborepo)
-bun run db:push      # Aplica el esquema de Prisma a la base de datos
-bun run db:studio    # Abre Prisma Studio
+pnpm install                   # Instala dependencias de todos los workspaces
+pnpm dev                       # Levanta la API (4000) y la web (6173) vía Turborepo
+pnpm typecheck                 # tsc --noEmit en todos los paquetes
+pnpm lint                      # biome check (backend y shared)
+pnpm --filter @gruperly/backend test      # vitest (backend)
+pnpm --filter @gruperly/backend db:push   # Aplica el esquema de Prisma a la base de datos
+pnpm --filter @gruperly/backend db:studio # Abre Prisma Studio
 ```
 
 ## Setup de base de datos
 
-1. Crea una base PostgreSQL y copia `apps/api/.env.example` → `apps/api/.env`, ajustando `DATABASE_URL`.
-2. Ejecuta `bun run db:generate` y `bun run db:push` (o `bun run db:migrate` en desarrollo).
+1. Crea una base PostgreSQL y copia `apps/backend/.env.example` → `apps/backend/.env`, ajustando `DATABASE_URL`.
+2. Ejecuta `pnpm --filter @gruperly/backend db:generate` y `pnpm --filter @gruperly/backend db:push` (o `pnpm --filter @gruperly/backend db:migrate` en desarrollo).