(bugfix) Validate Django-managed SSH private keys

Validate uploaded SSH private keys with ssh-keygen before saving them so
invalid, malformed, or unsupported key material is rejected in the
control panel instead of failing later during rsync.

Auto-populate the public key when it is omitted, add an edit flow for
existing SSH credentials, and cover create, update, and invalid-key
paths with view tests.

src/pobsync_server/urls.py

@@ -11,6 +11,7 @@ urlpatterns = [
     path("config/global/", views.edit_global_config, name="edit_global_config"),
     path("ssh-credentials/", views.ssh_credentials, name="ssh_credentials"),
     path("ssh-credentials/new/", views.create_ssh_credential, name="create_ssh_credential"),
+    path("ssh-credentials/<int:credential_id>/", views.edit_ssh_credential, name="edit_ssh_credential"),
     path("hosts/new/", views.create_host_config, name="create_host_config"),
     path("hosts/<str:host>/", views.host_detail, name="host_detail"),
     path("hosts/<str:host>/config/", views.edit_host_config, name="edit_host_config"),