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
Kanboard API returns int (ID) on success and false (bool) on failure
for create operations. Add IntOrFalse type that handles both cases
and update CreateTask, CreateComment, CreateTag, CreateTaskFile, and
CreateTaskLink to use it.
Some Kanboard versions return is_active and similar fields as
numeric 0/1 instead of string "0"/"1". Update UnmarshalJSON to
try string, then number, then bool parsing.
