Integration with systemd-creds

[mvfc]

Discussed in #50

^{Originally posted by msfjarvis January 13, 2026}
I want to use the backvault container in a NixOS system where secrets can be provided declaratively, so I would prefer to pass secrets in with environment variables using systemd-creds instead of manually navigating a Web UI.

Integrate with systemd-creds for credential retrieval if available before setting up the WebUI.