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/models.py

@@ -98,6 +98,13 @@ class Event(ClubScopedModel):
 
     max_referees = models.PositiveSmallIntegerField(_("max referees"), default=2, help_text=_("How many referees can be assigned to this game. Only meaningful for home games -- ignored otherwise."))
 
+    deadline_reminder_sent_at = models.DateTimeField(
+        _("deadline reminder sent at"),
+        null=True,
+        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."),
+    )
+
     class Meta:
         verbose_name = _("event")
         verbose_name_plural = _("events")