Let a coach schedule a line-up to publish itself later

Publish now still works exactly as before (default action), but a coach
can also pick a future date/time -- events.services.lineup.
schedule_lineup_publish sets Lineup.scheduled_publish_at, and a new
periodic task (events.tasks.publish_scheduled_lineups, every 15 minutes)
publishes it once that time arrives via the same publish_lineup used for a
manual publish, so selection/Attendance/notifications all work identically
either way. A pending schedule can be cancelled or published early from the
same screen.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ECGMEwrc2k4D8VQuwjstj9

events/models.py

@@ -211,6 +211,12 @@ class Lineup(UUIDModel):
     event = models.OneToOneField(Event, on_delete=models.CASCADE, related_name="lineup", verbose_name=_("event"))
     team = models.ForeignKey(Team, on_delete=models.CASCADE, related_name="lineups", verbose_name=_("team"))
     published_at = models.DateTimeField(_("published at"), null=True, blank=True)
+    scheduled_publish_at = models.DateTimeField(
+        _("scheduled publish at"),
+        null=True,
+        blank=True,
+        help_text=_("If set (and not yet published), this line-up publishes itself automatically at this time -- events.tasks.publish_scheduled_lineups is the periodic sweep that catches it."),
+    )
     created_by = models.ForeignKey(Member, on_delete=models.SET_NULL, null=True, blank=True, related_name="created_lineups", verbose_name=_("created by"))
 
     class Meta: