(refactor) Remove unused schedule user field

Drop the legacy schedule user setting from the Django model, form, defaults,
and configure command.

Schedules are executed by the pobsync scheduler service under the configured
systemd service user, while remote SSH login users are configured separately
on global or host backup config.

Add a migration to remove the unused database column and update schedule
view tests around the simplified form.

src/pobsync_backend/views.py

@@ -575,7 +575,6 @@ def _next_run_for_schedule(schedule: ScheduleConfig | None, host_config: HostCon
 def _default_schedule_initial() -> dict[str, object]:
     return {
         "cron_expr": "15 2 * * *",
-        "user": "root",
         "enabled": True,
         "prune_max_delete": 10,
     }