feat: add Vaultwarden module

- Add native NixOS Vaultwarden service module
- Supports WebSocket for real-time sync notifications
- Integrates with nginx via extraLocations for /notifications/hub
- Configurable signup, invitations, and SMTP settings
- Uses SOPS for admin token secret management

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

modules/default.nix

@@ -6,5 +6,6 @@
     ./nginx.nix
     ./searxng.nix
     ./openclaw-podman.nix
+    ./vaultwarden.nix
   ];
 }