feat: add GetTaskFile and RemoveAllTaskFiles methods

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

types.go

@@ -237,6 +237,8 @@ type TaskFile struct {
 	DateCreation Timestamp   `json:"date_creation"`
 	UserID       StringInt   `json:"user_id"`
 	Size         StringInt64 `json:"size"`
+	Username     string      `json:"username"`  // Only returned by getAllTaskFiles
+	UserName     string      `json:"user_name"` // Only returned by getAllTaskFiles
 }
 
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