- Tag entity: add parentId (UUID FK) and color (String) fields - TagUpdateDTO and TagTreeNodeDTO records - ErrorCode: INVALID_TAG_COLOR, TAG_CYCLE_DETECTED - TagRepository: findAncestorIds() recursive CTE query - TagService: cycle detection, color validation, getTagTree() - TagController: use TagUpdateDTO, add GET /api/tags/tree endpoint Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>