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/GeschichteListRow.svelte.spec.ts

@@ -12,7 +12,7 @@ const baseRow = (overrides = {}) => ({
 	body: '<p>Im Jahr 1923...</p>',
 	type: 'STORY' as 'STORY' | 'JOURNEY',
 	status: 'PUBLISHED' as 'PUBLISHED' | 'DRAFT',
-	author: { firstName: 'Anna', lastName: 'Schmidt', email: 'a@x' },
+	author: { firstName: 'Anna', lastName: 'Schmidt' },
 	publishedAt: '2026-04-15T10:00:00Z',
 	...overrides
 });