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

features/jobs.py

@@ -15,6 +15,11 @@ JOB_REGISTRY = {
         "description": _("Materialises recurring event occurrences up to the rolling horizon, so the calendar never runs dry."),
         "schedule": _("Daily at 03:00"),
     },
+    "events.tasks.send_deadline_reminders": {
+        "label": _("Send deadline reminders"),
+        "description": _("Nudges whoever still hasn't answered an event, one week before its answer deadline (or its start, when no deadline is set)."),
+        "schedule": _("Daily at 07:00"),
+    },
     "billing.tasks.renew_subscriptions": {
         "label": _("Renew subscriptions"),
         "description": _("Opens the next billing period for clubs whose current one is running out."),