(ops) Add terminal self-check command for native installs

Add check_pobsync_install so native deployments can run the same runtime
diagnostics from the terminal that are available in the Django Self Check view.

The command prints every check with status, returns a failing exit code when
install-critical checks fail, supports fail-on-warning for stricter automation,
and is documented in the installer output and README update flow.

README.md

@@ -137,6 +137,7 @@ loaded before Django starts:
 ```
 sudo -u pobsync pobsync-manage showmigrations pobsync_backend
 sudo -u pobsync pobsync-manage check
+sudo -u pobsync pobsync-manage check_pobsync_install
 ```
 
 The UI includes:
@@ -198,6 +199,7 @@ Then check:
 ```
 systemctl status pobsync-web pobsync-worker pobsync-scheduler
 sudo -u pobsync pobsync-manage check
+sudo -u pobsync pobsync-manage check_pobsync_install
 ```
 
 ## Development