(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_backend/tests/test_views.py

@@ -83,6 +83,17 @@ class ViewTests(TestCase):
         self.assertContains(response, reverse("create_host_config"))
         self.assertContains(response, "Add first host")
 
+    def test_self_check_renders_runtime_checks(self) -> None:
+        self.client.force_login(self.staff_user)
+
+        response = self.client.get(reverse("self_check"))
+
+        self.assertEqual(response.status_code, 200)
+        self.assertContains(response, "Self Check")
+        self.assertContains(response, "Django debug")
+        self.assertContains(response, "Database connection")
+        self.assertContains(response, "POBSYNC_HOME")
+
     def test_ssh_credentials_view_creates_key(self) -> None:
         self.client.force_login(self.staff_user)