Add encryption status to notification health endpoint (P2)

Backend:
- Add IsEncryptionEnabled() method to ConfigPersistence
- Include encryption status in /api/notifications/health response
- Allows frontend to warn when credentials are stored in plaintext

Frontend:
- Update NotificationHealth type to include encryption.enabled field
- Frontend can now display warnings when encryption is disabled

This addresses the P2 requirement for encryption visibility, allowing
operators to know when notification credentials are not encrypted at rest.

frontend-modern/src/api/notifications.ts

@@ -238,6 +238,9 @@ export class NotificationsAPI {
       total: number;
       enabled: number;
     };
+    encryption: {
+      enabled: boolean;
+    };
     healthy: boolean;
   }> {
     return apiFetchJSON(`${this.baseUrl}/health`);