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/documents/[id]/edit/+page.svelte

@@ -11,7 +11,7 @@ import SaveBar from './SaveBar.svelte';
 let { data, form } = $props();
 
 let { document: doc } = untrack(() => data);
-let tags = $state(doc.tags ? doc.tags.map((t: { name: string }) => t.name) : []);
+let tags = $state(doc.tags ?? []);
 let senderId = $state(doc.sender?.id ?? '');
 let selectedReceivers = $state(doc.receivers ?? []);
 </script>