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/new/+page.svelte

@@ -10,7 +10,7 @@ import { type FilenameParseResult } from '$lib/utils/filename';
 
 let { data, form } = $props();
 
-let tags: string[] = $state([]);
+let tags: { name: string; id?: string; color?: string; parentId?: string }[] = $state([]);
 let senderId = $state(untrack(() => data.initialSenderId));
 let selectedReceivers: { id: string; firstName?: string; lastName: string; displayName: string }[] =
 	$state(untrack(() => data.initialReceivers));