feat(api): scaffold Fastify server with health route

- ESM Fastify 4 with Pino logger (pretty-print in dev only)
- Plugins: helmet, cors (allowlist via CORS_ORIGINS), rate-limit, sensible
- Zod-validated environment config in src/config/env.ts
- Modular structure under src/modules with /health route
- tsx watch for dev, tsc for build, dist/server.js as production entry

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

apps/api/.env.example

@@ -0,0 +1,10 @@
+NODE_ENV=development
+PORT=4000
+HOST=127.0.0.1
+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