(ops) Install pobsync-manage for native management commands

Add a pobsync-manage wrapper that loads the native environment file before
running Django management commands, so production commands use the same
database and runtime settings as the systemd services.

Install the wrapper from the systemd installer, use it for migrations,
static collection, SSH key setup, and superuser creation, and document it
in the README for operational commands.

README.md

@@ -43,6 +43,7 @@ The installer will, by default:
 - copy the checkout to `/opt/pobsync/app`
 - create `/opt/pobsync/venv`
 - write `/etc/pobsync/pobsync.env` if it does not exist
+- install `pobsync-manage`, a Django management wrapper that loads `/etc/pobsync/pobsync.env`
 - create `/var/lib/pobsync`, `/var/log/pobsync`, and the backup root
 - install Python dependencies
 - run migrations and collect static files
@@ -127,7 +128,15 @@ http://127.0.0.1:8010/
 Create a superuser if needed:
 
 ```
-sudo -u pobsync /opt/pobsync/venv/bin/python /opt/pobsync/app/manage.py createsuperuser
+sudo -u pobsync pobsync-manage createsuperuser
+```
+
+For other Django management commands on native installs, use `pobsync-manage` so the production environment file is
+loaded before Django starts:
+
+```
+sudo -u pobsync pobsync-manage showmigrations pobsync_backend
+sudo -u pobsync pobsync-manage check
 ```
 
 The UI includes: