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/services/__init__.py

@@ -1,3 +1,22 @@
 from .attendance import effective_members, sync_event_attendances
+from .recurrence import (
+    apply_template,
+    cancel_occurrence,
+    detach_occurrence,
+    generate_occurrences,
+    horizon,
+    occurrence_datetimes,
+    propagate_series,
+)
 
-__all__ = ["effective_members", "sync_event_attendances"]
+__all__ = [
+    "apply_template",
+    "cancel_occurrence",
+    "detach_occurrence",
+    "effective_members",
+    "generate_occurrences",
+    "horizon",
+    "occurrence_datetimes",
+    "propagate_series",
+    "sync_event_attendances",
+]