feat(favorites): add EventLike model, favorites API, and notifications module

- EventLike model (user × event unique constraint, indexed)
- contributed_by field on Event (EVT ID or email of community contributor)
- Favorites API endpoints: toggle-like, my-likes, my-liked-events
- Notifications app wired into main urls.py at /api/notifications/
- accounts migration 0014_merge_0013 (resolves split 0013 branches)
- requirements.txt updated

notifications/apps.py

@@ -0,0 +1,6 @@
+from django.apps import AppConfig
+
+
+class NotificationsConfig(AppConfig):
+    default_auto_field = 'django.db.models.BigAutoField'
+    name = 'notifications'