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/models.py

@@ -45,6 +45,10 @@ class Event(ClubScopedModel):
         SOCIAL = "social", _("social")
         OTHER = "other", _("other")
 
+    series = models.ForeignKey("EventSeries", on_delete=models.CASCADE, related_name="occurrences", null=True, blank=True, verbose_name=_("series"), help_text=_("The recurring series this occurrence belongs to; blank for one-off events."))
+    detached = models.BooleanField(_("detached"), default=False, help_text=_("Edited independently; excluded from series-wide updates and regeneration."))
+    cancelled = models.BooleanField(_("cancelled"), default=False)
+
     teams = models.ManyToManyField(Team, related_name="scheduled_events", blank=True, verbose_name=_("teams"))
     invited_members = models.ManyToManyField(Member, related_name="invited_to_events", blank=True, verbose_name=_("invited members"))
     excluded_members = models.ManyToManyField(Member, related_name="excluded_from_events", blank=True, verbose_name=_("excluded members"))
@@ -70,6 +74,33 @@ class Event(ClubScopedModel):
         return self.title
 
 
+class EventSeries(ClubScopedModel):
+    """A recurring event definition that materialises concrete Event rows."""
+
+    rrule = models.CharField(_("recurrence rule"), max_length=255, help_text=_("RFC 5545 RRULE, e.g. FREQ=WEEKLY;BYDAY=MO,WE."))
+    dtstart = models.DateTimeField(_("first occurrence"))
+    duration = models.DurationField(_("duration"), null=True, blank=True, help_text=_("Length of each occurrence; sets each event's end."))
+    excluded_dates = models.JSONField(_("excluded dates"), default=list, blank=True, help_text=_("ISO start datetimes of occurrences removed from the series (EXDATEs)."))
+    generated_until = models.DateTimeField(_("generated until"), null=True, blank=True, help_text=_("Occurrences have been materialised up to this point."))
+
+    # Template copied onto each generated occurrence.
+    kind = models.CharField(_("kind"), max_length=10, choices=Event.EventKind.choices, default=Event.EventKind.OTHER)
+    title = models.CharField(_("title"), max_length=255)
+    location = models.ForeignKey(Location, on_delete=models.SET_NULL, related_name="event_series", null=True, blank=True, verbose_name=_("location"))
+    opponent = models.ForeignKey(Opponent, on_delete=models.SET_NULL, related_name="event_series", null=True, blank=True, verbose_name=_("opponent"))
+    teams = models.ManyToManyField(Team, related_name="event_series", blank=True, verbose_name=_("teams"))
+    invited_members = models.ManyToManyField(Member, related_name="invited_to_event_series", blank=True, verbose_name=_("invited members"))
+    excluded_members = models.ManyToManyField(Member, related_name="excluded_from_event_series", blank=True, verbose_name=_("excluded members"))
+
+    class Meta:
+        verbose_name = _("event series")
+        verbose_name_plural = _("event series")
+        ordering = ["title"]
+
+    def __str__(self):
+        return self.title
+
+
 class Attendance(UUIDModel):
     class AttendanceStatus(models.TextChoices):
         PRESENT = "present", _("present")