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/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;