feat(api): integrate Prisma client with /db/health endpoint

- Add @ketopath/db and fastify-plugin to apps/api
- prismaPlugin decorates the Fastify instance with prisma and disconnects
  on app close, with FastifyInstance type augmented in src/types/fastify.d.ts
- DATABASE_URL is now required by env validation
- New module modules/db with GET /db/health running SELECT 1 via Prisma
- dev script switched to tsx --env-file=.env for env loading

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/api/.env.example

@@ -6,5 +6,4 @@ LOG_LEVEL=info
 # CORS — origini consentite (separate da virgola)
 CORS_ORIGINS=http://localhost:3000
 
-# Database (configurato in step successivo con Prisma)
-# DATABASE_URL=postgresql://user:password@localhost:5432/ketopath
+DATABASE_URL=postgresql://luciano:luciano%40@localhost:5432/ketopath?schema=public