feat(faz3): implement file upload with Cloudflare R2

- Add AWS SDK for S3-compatible R2 storage
- Create R2 client with presigned URL support
- Add attachment upload/download/delete API routes
- Create FileUpload and AttachmentList components
- Integrate file upload in message form (new & edit)
- Add .env.example with R2 configuration

package.json

@@ -10,6 +10,8 @@
   },
   "dependencies": {
     "@auth/prisma-adapter": "^2.11.1",
+    "@aws-sdk/client-s3": "^3.958.0",
+    "@aws-sdk/s3-request-presigner": "^3.958.0",
     "@prisma/adapter-pg": "^7.2.0",
     "@prisma/client": "^7.2.0",
     "@radix-ui/react-label": "^2.1.8",