fix(geschichte): stop exposing author email in the list projection

GET /api/geschichten shipped every author's AppUser email to all readers via
GeschichteSummary.AuthorSummary — contradicting the documented rule that
author projections never expose email or group memberships. The frontend
only used it as a display-name fallback; it now falls back to [Unbekannt],
matching the server-side rule in GeschichteService.toView.

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

frontend/src/lib/geschichte/GeschichtenCard.svelte

@@ -27,7 +27,7 @@ function authorName(g: GeschichteSummary): string {
 	const a = g.author;
 	if (!a) return '';
 	const full = [a.firstName, a.lastName].filter(Boolean).join(' ').trim();
-	return full || a.email || '';
+	return full || '[Unbekannt]';
 }
 </script>