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

package.json

@@ -3,22 +3,24 @@
   "version": "0.1.0",
   "private": true,
   "type": "module",
-  "workspaces": [
-    "apps/*",
-    "packages/*"
-  ],
   "scripts": {
     "dev": "turbo dev",
     "build": "turbo build",
     "lint": "turbo lint",
+    "lint:fix": "turbo lint:fix",
     "typecheck": "turbo typecheck",
-    "db:generate": "bun run --filter @gruperly/api db:generate",
-    "db:migrate": "bun run --filter @gruperly/api db:migrate",
-    "db:push": "bun run --filter @gruperly/api db:push",
-    "db:studio": "bun run --filter @gruperly/api db:studio"
+    "test": "pnpm --filter @gruperly/backend test",
+    "db:generate": "pnpm --filter @gruperly/backend db:generate",
+    "db:migrate": "pnpm --filter @gruperly/backend db:migrate",
+    "db:push": "pnpm --filter @gruperly/backend db:push",
+    "db:studio": "pnpm --filter @gruperly/backend db:studio"
   },
   "devDependencies": {
+    "@biomejs/biome": "^2.4.15",
     "turbo": "^2.3.3"
   },
-  "packageManager": "bun@1.4.0"
-}
+  "packageManager": "pnpm@9.0.0",
+  "engines": {
+    "node": ">=20"
+  }
+}