Schedule Nextcloud's cron.php in the user's crontab, not a systemd timer

The timer was justified partly on XDG_RUNTIME_DIR: a user crontab "would need it
faked into every line". That is wrong here. The caddy playbook enables lingering,
so /run/user/<uid> exists whether or not anyone is logged in, and that is where
podman keeps the state `exec` reads. Nothing needs to be set.

The one real constraint is that the job cannot go in root's crontab: root cannot
`podman exec` into another user's rootless containers. This user's crontab is
fine, and the play runs as that user, so `ansible.builtin.cron` with no `user:`
writes exactly there.

That replaces four tasks and two template files with one task and no files, and it
is what the service had before the Quadlet conversion. The cost is that cron sends
its output to local mail instead of the journal; the readme says so and names the
timer as the alternative.

While here, the XDG_RUNTIME_DIR asides come out of the comments that carried them:
the play runs over SSH, so `systemctl --user` works because systemd's user manager
is reachable over that connection, and that is all a reader needs to know.

nextcloud/roles/postgres/handlers/main.yaml

@@ -1,7 +1,7 @@
 ---
 # Postgres is its own playbook (postgres/playbook.yml) and a rootless Quadlet
-# unit there. This handler runs as the connection user, so `systemctl --user`
-# works without setting XDG_RUNTIME_DIR.
+# unit there. This handler runs as the connection user, which is what makes
+# `systemctl --user` work here.
 #
 # Required by postgres/roles/attach, which notifies it whenever this app's
 # drop-in changes: a new drop-in changes the generated unit, so Postgres must be