Mirror Form990LineId: add Form1099BoxId + Form1099BoxCode to all four
category DTOs (response + request, group + sub); load a boxCodes lookup
dictionary in GetAllAsync and project it; set/copy the field in
CreateGroupAsync, UpdateGroupAsync, CreateSubCategoryAsync, and
UpdateSubCategoryAsync. All 4 category-service unit tests pass.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>