(feature) Add full native installer and self-check page

Expand the systemd installer so it can perform a complete native
installation with sensible defaults: copy the checkout into the target
app directory, create runtime directories, write the environment file,
install dependencies, configure systemd units, and optionally configure
nginx.

Add a staff-only Django self-check page that verifies runtime settings,
required binaries, writable paths, database connectivity, global config
state, and systemd service status when available.

Document installer overrides and expose the self-check from the main
navigation.

src/pobsync_server/urls.py

@@ -8,6 +8,7 @@ from pobsync_backend import api, views
 
 urlpatterns = [
     path("", views.dashboard, name="dashboard"),
+    path("self-check/", views.self_check, name="self_check"),
     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"),