Backend:
- TagRepository: add findDescendantIdsByName() recursive CTE query
- TagService: add expandTagNamesToDescendantIdSets() for document search
Frontend:
- TagInput: accept Tag[] (id, name, color, parentId) instead of string[]
- Chips show color dot via var(--c-tag-{color}) when tag has color
- Suggestions grouped hierarchically: children indented under their parents
- Update DescriptionSection, edit/new pages, SearchFilterBar, +page.svelte
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>