feat: declarative password salt, secret config

[karaolidis]

Summary

Adds --password-salt and --secret options to maintenance:install for fully declarative deployments (e.g., sops-nix, Ansible Vault). Values are validated for minimum length and fall back to random generation if not provided.

Running this in my homelab for months without issues.

TODO

I have not added unit tests to this commit as the main change is in the install function that has a lot of side effects. Looking for comments in regards to how to implement this, if at all.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)