feat(events): series-level gathering and deadline offsets

EventSeries gains gathering_offset and deadline_offset (durations before
the start). Each generated occurrence derives its gathering/deadline from
them (and clears them when unset); propagate_series pushes offset changes
to non-detached future occurrences. Surface them in the admin. 100%
coverage.

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

events/migrations/0006_event_series_offsets.py

@@ -0,0 +1,23 @@
+# Generated by Django 6.0.6 on 2026-07-12 16:03
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('events', '0005_event_series'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='eventseries',
+            name='deadline_offset',
+            field=models.DurationField(blank=True, help_text="How long before the start each occurrence's sign-up deadline is.", null=True, verbose_name='deadline offset'),
+        ),
+        migrations.AddField(
+            model_name='eventseries',
+            name='gathering_offset',
+            field=models.DurationField(blank=True, help_text="How long before the start each occurrence's gathering time is.", null=True, verbose_name='gathering offset'),
+        ),
+    ]