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

accounts/migrations/0014_merge_0013.py

@@ -0,0 +1,13 @@
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+    """Merge migration to resolve conflicting 0013 migrations."""
+
+    dependencies = [
+        ('accounts', '0013_merge_eventify_id'),
+        ('accounts', '0013_user_district_changed_at'),
+    ]
+
+    operations = [
+    ]