feat(#221): change TagInput binding to Tag[], add color dots and hierarchy grouping

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>

frontend/src/routes/enrich/[id]/+page.svelte

@@ -33,7 +33,7 @@ $effect(() => {
 onDestroy(() => fileLoader.destroy());
 
 // Form state
-let tags = $state(untrack(() => doc.tags?.map((t: { name: string }) => t.name) ?? []));
+let tags = $state(untrack(() => doc.tags ?? []));
 let senderId = $state(untrack(() => doc.sender?.id ?? ''));
 let selectedReceivers = $state(untrack(() => doc.receivers ?? []));
 </script>