feat: implement secure file upload system with JWT authentication

- Add JWT-based secure file access for local storage with 1-hour expiry
- Implement GORM repository methods for attachment CRUD operations
- Add secure file serving endpoint with token validation
- Update storage interface to support user context in URL generation
- Add comprehensive security features including path traversal protection
- Update documentation with security model and configuration examples
- Add utility functions for hex/byte conversion and UUID validation
- Configure secure file permissions (0600) for uploaded files

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

config.storage.json.sample

@@ -9,7 +9,8 @@
     "backend": "local",
     "local": {
       "root_dir": "./uploads",
-      "base_url": "https://yourapp.com/files"
+      "base_url": "https://yourapp.com/files",
+      "signing_key": "your-secret-jwt-signing-key-change-this-in-production"
     }
   }
 }