(feature) Add Django-managed SSH credentials

Add SSH credentials as first-class Django data so backup keys can be
uploaded through the control panel instead of mounted into containers.

Credentials can be selected globally or overridden per host. At runtime
the selected key is materialized inside the container with restrictive
file permissions and injected into the rsync SSH command via IdentityFile.
Known hosts entries are handled the same way when configured.

Add control panel views for creating and listing SSH keys, expose the
fields in config forms and admin, document the workflow, and cover global
and host credential selection with tests.

src/pobsync_server/urls.py

@@ -9,6 +9,8 @@ from pobsync_backend import api, views
 urlpatterns = [
     path("", views.dashboard, name="dashboard"),
     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("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"),