(feature) Add host doctor checks and Django log viewer

Add host-level checks for address, enabled state, SSH credential
selection, and backup directory readiness, and show them on the host
detail page.

Create host backup directories during host creation and prefill new
hosts from the default global config.

Add a staff-only logs view backed by journalctl with filtering by
pobsync unit, priority, and message text.

Improve runtime checks for gunicorn in virtualenv installs and ensure
the native installer grants the service user access to the backup root.

src/pobsync_server/urls.py

@@ -9,6 +9,7 @@ from pobsync_backend import api, views
 urlpatterns = [
     path("", views.dashboard, name="dashboard"),
     path("self-check/", views.self_check, name="self_check"),
+    path("logs/", views.logs, name="logs"),
     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"),