cleanup: remove unused handlers and add integration test for unused handlers

Removed obsolete handlers from roles (VirtualBox, backup-to-USB, OpenLDAP)
and introduced an integration test under tests/integration/test_handlers_invoked.py
that ensures all handlers defined in roles/*/handlers are actually notified
somewhere in the code base. This keeps the repository clean by preventing
unused or forgotten handlers from accumulating.

Ref: https://chatgpt.com/share/68b6b28e-4388-800f-87d2-34dfb34b8d36

roles/svc-bkp-loc-2-usb/handlers/main.yml

@@ -1,6 +0,0 @@
-
-- name: "reload svc-bkp-loc-2-usb service"
-  systemd:
-    name: "{{ 'svc-bkp-loc-2-usb' | get_service_name(SOFTWARE_NAME) }}"
-    state: reloaded
-    daemon_reload: yes