feat(geschichten): wire the ?documentId list filter the drawer already links to

DocumentMetadataDrawer links to /geschichten?documentId={id}, but the list
loader silently dropped the param — the user got the unfiltered list. The
loader now validates the UUID and forwards it to GET /api/geschichten,
returning it as documentIdFilter in page data.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/routes/geschichten/+page.server.ts

@@ -6,16 +6,21 @@ import type { PageServerLoad } from './$types';
 
 type Person = components['schemas']['Person'];
 
+const UUID_PATTERN = /^[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$/i;
+
 export const load: PageServerLoad = async ({ url, fetch }) => {
 	const api = createApiClient(fetch);
 	const personIds = url.searchParams.getAll('personId');
+	const rawDocumentId = url.searchParams.get('documentId');
+	const documentId = rawDocumentId && UUID_PATTERN.test(rawDocumentId) ? rawDocumentId : null;
 
 	const [listResult, ...personResults] = await Promise.all([
 		api.GET('/api/geschichten', {
 			params: {
 				query: {
 					status: 'PUBLISHED',
-					personId: personIds.length ? personIds : undefined
+					personId: personIds.length ? personIds : undefined,
+					documentId: documentId ?? undefined
 				}
 			}
 		}),
@@ -32,6 +37,7 @@ export const load: PageServerLoad = async ({ url, fetch }) => {
 
 	return {
 		geschichten: listResult.data ?? [],
-		personFilters
+		personFilters,
+		documentIdFilter: documentId
 	};
 };