refactor(document-thumbnail): read doc.thumbnailUrl instead of composing locally

The backend now exposes thumbnailUrl as a serialised computed property
on Document, so the component drops its dependency on the frontend
URL-builder. PersonDocumentList's inline Doc prop type follows the
same shift (#309).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/lib/components/DocumentThumbnail.svelte

@@ -1,14 +1,10 @@
 <script lang="ts">
 import type { components } from '$lib/generated/api';
-import { thumbnailUrl } from '$lib/thumbnails';
 
-type Doc = Pick<
-	components['schemas']['Document'],
-	'id' | 'thumbnailKey' | 'thumbnailGeneratedAt' | 'contentType'
->;
+type Doc = Pick<components['schemas']['Document'], 'id' | 'thumbnailUrl' | 'contentType'>;
 
 let { doc, size = 'sm' }: { doc: Doc; size?: 'sm' | 'lg' } = $props();
-const url = $derived(thumbnailUrl(doc));
+const url = $derived(doc.thumbnailUrl ?? null);
 
 const containerClass = $derived(
 	size === 'lg'

frontend/src/routes/persons/[id]/PersonDocumentList.svelte

@@ -20,8 +20,7 @@ let {
 		location?: string | null;
 		status: string;
 		contentType?: string;
-		thumbnailKey?: string;
-		thumbnailGeneratedAt?: string;
+		thumbnailUrl?: string;
 	}[];
 	heading: string;
 	emptyMessage: string;