feat: add source field with 3 options, fix EventListAPI fallback, add is_eventify_event to API response

- Event.source field updated: eventify, community, partner (radio select in form)
- EventListAPI: fallback to all events when pincode returns < 6
- EventListAPI: include is_eventify_event and source in serializer
- Admin API: add source to list serializer
- Django admin: source in list_display, list_filter, list_editable
- Event form template: proper radio button rendering for source field

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

events/migrations/migrations/0006_alter_event_source.py

@@ -0,0 +1,18 @@
+# Generated by Django 5.0 on 2025-12-19 22:33
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('events', '0005_event_source'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='event',
+            name='source',
+            field=models.CharField(blank=True, choices=[('official', 'Official'), ('community', 'Community')], max_length=250),
+        ),
+    ]