(feature) Generate filesystem-backed SSH credentials

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.

src/pobsync_backend/config_source.py

@@ -11,6 +11,7 @@ from pobsync.paths import PobsyncPaths
 
 from .config_repository import global_config_data, host_config_data
 from .models import GlobalConfig, HostConfig, SshCredential
+from .ssh_keys import identity_path
 
 
 class DjangoConfigSource:
@@ -48,9 +49,12 @@ def _materialize_credential(credential: SshCredential) -> dict[str, str]:
     credential_dir.mkdir(mode=0o700, parents=True, exist_ok=True)
     os.chmod(credential_dir, 0o700)
 
-    identity_file = credential_dir / "identity"
-    identity_file.write_text(_with_trailing_newline(credential.private_key), encoding="utf-8")
-    os.chmod(identity_file, 0o600)
+    identity_file = identity_path(credential)
+    if credential.key_path:
+        os.chmod(identity_file, 0o600)
+    else:
+        identity_file.write_text(_with_trailing_newline(credential.private_key), encoding="utf-8")
+        os.chmod(identity_file, 0o600)
 
     result = {"identity_file": str(identity_file)}
     if credential.known_hosts.strip():