Add a periodic deadline-reminder job for events, including recurring series

New events.tasks.send_deadline_reminders (daily beat schedule, registered
in features.jobs.JOB_REGISTRY for the control panel's Jobs tab), the gap
notify_new_event's own docstring flagged: a recurring series' occurrences
never go through that on-creation path (they're bulk-generated by
extend_event_series, and notifying per-occurrence there would flood
everyone), so they never got any "you need to answer this" nudge at all.

This sweeps every upcoming event once, one week before whichever cutoff
matters -- the event's own answer deadline, or its start when none is
set -- and notifies whoever's still NO_RESPONSE, using the same
notify_members() -> Notification -> push chain notify_new_event already
uses. New Event.deadline_reminder_sent_at makes it idempotent: each
event's window opens once, is marked processed regardless of whether
anyone needed notifying, and a missed run still catches anything whose
window hasn't fully closed by the time the job next runs.

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

events/migrations/0023_event_deadline_reminder_sent_at.py

@@ -0,0 +1,18 @@
+# Generated by Django 6.0.6 on 2026-08-21 15:28
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('events', '0022_event_club_wide_event_groups_eventseries_club_wide_and_more'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='event',
+            name='deadline_reminder_sent_at',
+            field=models.DateTimeField(blank=True, help_text="Set once events.tasks.send_deadline_reminders has nudged whoever still hasn't answered -- keeps that job from reminding the same event twice.", null=True, verbose_name='deadline reminder sent at'),
+        ),
+    ]