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/migrations/0005_event_series.py

@@ -0,0 +1,57 @@
+# Generated by Django 6.0.6 on 2026-07-12 15:47
+
+import django.db.models.deletion
+import uuid
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('club', '0008_alter_clubmembership_unique_together_and_more'),
+        ('events', '0004_remove_event_team'),
+        ('members', '0002_alter_familymembership_unique_together_and_more'),
+        ('teams', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='event',
+            name='cancelled',
+            field=models.BooleanField(default=False, verbose_name='cancelled'),
+        ),
+        migrations.AddField(
+            model_name='event',
+            name='detached',
+            field=models.BooleanField(default=False, help_text='Edited independently; excluded from series-wide updates and regeneration.', verbose_name='detached'),
+        ),
+        migrations.CreateModel(
+            name='EventSeries',
+            fields=[
+                ('id', models.UUIDField(default=uuid.uuid4, editable=False, primary_key=True, serialize=False)),
+                ('rrule', models.CharField(help_text='RFC 5545 RRULE, e.g. FREQ=WEEKLY;BYDAY=MO,WE.', max_length=255, verbose_name='recurrence rule')),
+                ('dtstart', models.DateTimeField(verbose_name='first occurrence')),
+                ('duration', models.DurationField(blank=True, help_text="Length of each occurrence; sets each event's end.", null=True, verbose_name='duration')),
+                ('excluded_dates', models.JSONField(blank=True, default=list, help_text='ISO start datetimes of occurrences removed from the series (EXDATEs).', verbose_name='excluded dates')),
+                ('generated_until', models.DateTimeField(blank=True, help_text='Occurrences have been materialised up to this point.', null=True, verbose_name='generated until')),
+                ('kind', models.CharField(choices=[('training', 'training'), ('match', 'match'), ('tournament', 'tournament'), ('meeting', 'meeting'), ('social', 'social'), ('other', 'other')], default='other', max_length=10, verbose_name='kind')),
+                ('title', models.CharField(max_length=255, verbose_name='title')),
+                ('club', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='%(class)ss', to='club.club')),
+                ('excluded_members', models.ManyToManyField(blank=True, related_name='excluded_from_event_series', to='members.member', verbose_name='excluded members')),
+                ('invited_members', models.ManyToManyField(blank=True, related_name='invited_to_event_series', to='members.member', verbose_name='invited members')),
+                ('location', models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='event_series', to='events.location', verbose_name='location')),
+                ('opponent', models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='event_series', to='events.opponent', verbose_name='opponent')),
+                ('teams', models.ManyToManyField(blank=True, related_name='event_series', to='teams.team', verbose_name='teams')),
+            ],
+            options={
+                'verbose_name': 'event series',
+                'verbose_name_plural': 'event series',
+                'ordering': ['title'],
+            },
+        ),
+        migrations.AddField(
+            model_name='event',
+            name='series',
+            field=models.ForeignKey(blank=True, help_text='The recurring series this occurrence belongs to; blank for one-off events.', null=True, on_delete=django.db.models.deletion.CASCADE, related_name='occurrences', to='events.eventseries', verbose_name='series'),
+        ),
+    ]