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/admin.py

@@ -0,0 +1,10 @@
+from django.contrib import admin
+from .models import Notification
+
+
+@admin.register(Notification)
+class NotificationAdmin(admin.ModelAdmin):
+    list_display = ('title', 'user', 'notification_type', 'is_read', 'created_at')
+    list_filter = ('notification_type', 'is_read', 'created_at')
+    search_fields = ('title', 'message', 'user__email')
+    readonly_fields = ('created_at',)