Add filesystem-backed SSH credentials for the native systemd deployment
path. Generated keys are stored below POBSYNC_HOME with 0600
permissions, while Django keeps the public key, fingerprint, path, and
selection metadata.
Add a Django SSH key generation view, delete action for unused generated
keys, and a management command used by the installer to ensure a default
backup key exists.
Update runtime config to use generated key paths directly as IdentityFile,
extend host checks to verify key readability, and keep legacy uploaded
keys available for compatibility.
