feat(events): recurring event series with occurrence sync

Add EventSeries (club-scoped): an RFC-5545 rrule + dtstart + duration and a
template (kind/title/location/opponent + audience M2M). Concrete Event rows
are materialised occurrences carrying a series FK plus detached/cancelled
flags; the series tracks excluded_dates (EXDATEs) and a generated_until
horizon watermark.

Recurrence service:
- occurrence_datetimes expands the rrule (via python-dateutil) up to a
  horizon, minus EXDATEs.
- generate_occurrences materialises missing rows, copies the template +
  audience (so attendance syncs through the existing signals), and is
  idempotent.
- cancel_occurrence adds an EXDATE and deletes (or soft-cancels) one
  occurrence so it isn't regenerated; detach_occurrence marks an occurrence
  as independently edited; propagate_series re-applies the template to
  non-detached future occurrences.
- extend_event_series management command rolls the horizon forward.

Register EventSeries in the admin and surface series/detached/cancelled on
the Event admin. Add python-dateutil. Full suite at 100% coverage.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

events/management/commands/extend_event_series.py

@@ -0,0 +1,20 @@
+from django.core.management.base import BaseCommand
+
+from events.models import EventSeries
+from events.services import generate_occurrences, horizon
+
+
+class Command(BaseCommand):
+    help = "Materialise recurring event occurrences up to the rolling horizon."
+
+    def handle(self, *args, **options):
+        until = horizon()
+        total = 0
+
+        for series in EventSeries.objects.all():
+            created = generate_occurrences(series, until)
+            total += len(created)
+            if created:
+                self.stdout.write(f"{series}: generated {len(created)} occurrence(s).")
+
+        self.stdout.write(self.style.SUCCESS(f"Done. Generated {total} occurrence(s) across {EventSeries.objects.count()} series."))