feat(events): formal end date for recurring series

Add EventSeries.until: an explicit series end that caps occurrence
generation (whichever comes first — it, the generation horizon, or the
rule's own COUNT/UNTIL). Blank means open-ended. Surface it in the admin.
100% coverage.

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

events/migrations/0007_event_series_until.py

@@ -0,0 +1,18 @@
+# Generated by Django 6.0.6 on 2026-07-12 17:21
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('events', '0006_event_series_offsets'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='eventseries',
+            name='until',
+            field=models.DateTimeField(blank=True, help_text="Series end: no occurrences are generated after this. Leave blank for open-ended (bounded by the rule's own COUNT/UNTIL, if any).", null=True, verbose_name='until'),
+        ),
+    ]