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/models.py

@@ -49,9 +49,10 @@ class Event(models.Model):
     important_information = models.TextField(blank=True)
     venue_name = models.CharField(max_length=250, blank=True)
 
-    source = models.CharField(max_length=250, blank=True, choices=[
-        ('official', 'Official'),
-        ('community', 'Community'),
+    source = models.CharField(max_length=50, default='eventify', choices=[
+        ('eventify', 'Added by Eventify'),
+        ('community', 'Community Contribution'),
+        ('partner', 'Partner Event'),
     ])
 
     is_featured = models.BooleanField(default=False, help_text='Show this event in the featured section')