Complete the Task File API implementation with:
- GetTaskFile: retrieve single file metadata by ID
- RemoveAllTaskFiles: remove all files attached to a task
- Add Username/UserName fields to TaskFile struct
- Add TaskScope convenience methods: GetFile, RemoveFile,
DownloadFile, RemoveAllFiles
- Comprehensive tests for all new methods
Closes: kanboard-amh
- GetAllTaskFiles: retrieve all files attached to a task
- CreateTaskFile: upload file with base64 encoding
- DownloadTaskFile: download file with base64 decoding
- RemoveTaskFile: delete a task file
- TaskScope.GetFiles and UploadFile for fluent API
- Automatic base64 encoding/decoding for file content
- Comprehensive test coverage
