Add Clear button to notification bell dropdown

- Add notifications_cleared_at column to users table
- Clear button marks notifications as seen without deleting history
- Badge and dropdown only show notifications after last clear
- History page still shows all notifications

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/api/client.ts

@@ -313,6 +313,9 @@ export const notificationsApi = {
     api.get<{ count: number }>('/notifications/count', {
       params: hours ? { hours } : undefined,
     }),
+
+  clear: () =>
+    api.post<{ message: string }>('/notifications/clear'),
 };
 
 // Admin API