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.
Canonicalize uploaded OpenSSH private keys before validation by
normalizing line endings, removing whitespace from the base64 body,
and re-wrapping it between the BEGIN and END markers.
Add SSH credential tests that generate a real ed25519 key, damage its
wrapping, and verify that validation succeeds after normalization.
Return a clearer validation error for PEM private keys, which are not
supported by the current credential flow.
