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/lib/components/document/DescriptionSection.svelte

@@ -1,10 +1,10 @@
 <script lang="ts">
 import { untrack } from 'svelte';
-import TagInput from '$lib/components/TagInput.svelte';
+import TagInput, { type Tag } from '$lib/components/TagInput.svelte';
 import { m } from '$lib/paraglide/messages.js';
 
 let {
-	tags = $bindable<string[]>([]),
+	tags = $bindable<Tag[]>([]),
 	initialTitle = '',
 	initialDocumentLocation = '',
 	initialSummary = '',
@@ -12,7 +12,7 @@ let {
 	suggestedTitle = '',
 	hideTitle = false
 }: {
-	tags?: string[];
+	tags?: Tag[];
 	initialTitle?: string;
 	initialDocumentLocation?: string;
 	initialSummary?: string;
@@ -74,7 +74,7 @@ let titleValue = $derived(titleDirty ? titleOverride : suggestedTitle || titleOv
 		<div>
 			<p class="mb-1 block text-sm font-medium text-ink-2">{m.form_label_tags()}</p>
 			<TagInput bind:tags={tags} />
-			<input type="hidden" name="tags" value={tags.join(',')} />
+			<input type="hidden" name="tags" value={tags.map((t) => t.name).join(',')} />
 		</div>
 
 		<!-- Inhalt -->