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/migrations/0027_lineup_scheduled_publish_at.py

@@ -0,0 +1,18 @@
+# Generated by Django 6.0.6 on 2026-08-22 18:47
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('events', '0026_refereesignup'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='lineup',
+            name='scheduled_publish_at',
+            field=models.DateTimeField(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.', null=True, verbose_name='scheduled publish at'),
+        ),
+    ]