debusine-server should order itself against a postgresql running on the same machine

When rebooting a machine that hosts both debusine-server and the postgresql instance, systemd will not currently order these two services and shut them down concurrently. As a result, a typical shutdown procedure contains a lengthy traceback indicating that a database connection was closed unexpectedly.

During handling of the above exception, another exception occurred:
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/django/db/backends/utils.py", line 84, in _execute
    return self.cursor.execute(sql, params)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
psycopg2.OperationalError: server closed the connection unexpectedly
        This probably means the server terminated abnormally
        before or while processing the request.
The above exception was the direct cause of the following exception:

It seems likely that debusine meant to persist state into the database and that this state is lost as a result of the unexpected shutdown concurrency.

I suggest adding an After= relation at the right spot. On its own, it does not impose a dependency on the target unit, so running with an external database server is not impacted. The placement and value of this relation are more tricky. PostgreSQL uses instances units of the form postgresql@VERSION-CLUSTER.service and it is not entirely clear how to correctly capture them all or whether using the meta-unit postgresql.service is sufficient.