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/api/response.py

@@ -0,0 +1,111 @@
+"""
+Unified API Response Format Utilities
+
+This module provides helper functions to ensure all API endpoints
+return a consistent response format.
+"""
+
+from rest_framework.response import Response
+from rest_framework import status
+
+
+def success_response(data=None, message=None, status_code=status.HTTP_200_OK):
+    """
+    Create a standardized success response.
+    
+    Args:
+        data: Response data (dict, list, or any serializable object)
+        message: Optional human-readable success message
+        status_code: HTTP status code (default: 200)
+    
+    Returns:
+        Response: DRF Response object with unified format
+    
+    Example:
+        return success_response(
+            data={"id": 1, "name": "Example"},
+            message="Resource created successfully"
+        )
+    """
+    response_data = {
+        "success": True,
+        "data": data,
+    }
+    
+    if message:
+        response_data["message"] = message
+    
+    return Response(response_data, status=status_code)
+
+
+def error_response(
+    error,
+    errors=None,
+    status_code=status.HTTP_400_BAD_REQUEST,
+    message=None
+):
+    """
+    Create a standardized error response.
+    
+    Args:
+        error: Top-level error message (string)
+        errors: Optional field-specific validation errors (dict)
+        status_code: HTTP status code (default: 400)
+        message: Optional additional message (deprecated, use error)
+    
+    Returns:
+        Response: DRF Response object with unified format
+    
+    Example:
+        return error_response(
+            error="Validation failed",
+            errors={"email": ["Invalid email format"]},
+            status_code=status.HTTP_400_BAD_REQUEST
+        )
+    """
+    response_data = {
+        "success": False,
+        "error": error,
+    }
+    
+    if errors:
+        response_data["errors"] = errors
+    
+    # Backward compatibility: if message is provided, use it as error
+    if message and not error:
+        response_data["error"] = message
+    
+    return Response(response_data, status=status_code)
+
+
+def paginated_response(paginated_data, message=None):
+    """
+    Create a standardized paginated response.
+    
+    This wraps DRF's pagination response to include success flag
+    and optional message while preserving pagination metadata.
+    
+    Args:
+        paginated_data: DRF paginated response data (dict with count, next, previous, results)
+        message: Optional human-readable message
+    
+    Returns:
+        Response: DRF Response object with unified format
+    
+    Example:
+        paginator = CustomPageNumberPagination()
+        page = paginator.paginate_queryset(queryset, request)
+        serializer = MySerializer(page, many=True)
+        paginated_data = paginator.get_paginated_response(serializer.data).data
+        return paginated_response(paginated_data, message="Keywords retrieved successfully")
+    """
+    response_data = {
+        "success": True,
+        **paginated_data  # Unpack count, next, previous, results
+    }
+    
+    if message:
+        response_data["message"] = message
+    
+    return Response(response_data)
+