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.spec.ts

@@ -16,7 +16,6 @@ const makeStory = (id: string, title: string, body: string | undefined = '<p>Bod
 	items: [],
 	author: {
 		id: 'u1',
-		email: 'marcel@example.com',
 		firstName: 'Marcel',
 		lastName: 'Raddatz',
 		enabled: true,