feat(events): event owner and cross-club validation

Add Event.created_by (the owner, used later by the access service to let an
event's creator edit it).

Validate that an event's season/location/opponent — and an EventSeries'
location/opponent — belong to the event's club. The teams M2M cannot be checked
in clean() (M2M rows are written after save), so an m2m_changed pre_add receiver
rejects teams from another club.

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

events/migrations/0008_event_created_by.py

@@ -0,0 +1,20 @@
+# Generated by Django 6.0.6 on 2026-07-12 21:43
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('events', '0007_event_series_until'),
+        ('members', '0002_alter_familymembership_unique_together_and_more'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='event',
+            name='created_by',
+            field=models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='created_events', to='members.member', verbose_name='created by'),
+        ),
+    ]