feat(api): add unified response format utilities (Section 1, Step 1.1-1.3)

- Create response.py with success_response(), error_response(), paginated_response()
- Add unit tests for all response utility functions
- Create /api/ping/ health check endpoint using new format
- Update __init__.py to export response functions
- All changes are non-breaking - existing code unaffected

This implements Section 1 Task 1 from API-IMPLEMENTATION-PLAN-SECTION1.md
Ready for testing before applying to existing endpoints.

Ref: unified-api/API-IMPLEMENTATION-PLAN-SECTION1.md

backend/igny8_core/urls.py

@@ -16,9 +16,11 @@ Including another URLconf
 """
 from django.contrib import admin
 from django.urls import path, include
+from igny8_core.api.views import PingView
 
 urlpatterns = [
     path('admin/', admin.site.urls),
+    path('api/ping/', PingView.as_view(), name='ping'),  # Health check endpoint
     path('api/v1/auth/', include('igny8_core.auth.urls')),  # Auth endpoints
     path('api/v1/planner/', include('igny8_core.modules.planner.urls')),
     path('api/v1/writer/', include('igny8_core.modules.writer.urls')),