feat(core): add core module with mixins, services, and background jobs

- Add Model Mixins: TimestampMixin, SoftDeleteMixin, AuditMixin, etc.
- Add Base Services: BaseService, BulkOperationsMixin, QueryOptimizerMixin
- Add Base ViewSets with bulk operations
- Add BackgroundJob model for Celery task tracking
- Add BaseAppCommand for management commands
- Add permissions, pagination, filters, cache, logging
- Migrate tests to factory_boy + faker
- Add CHANGELOG.md
- 297 tests passing

src/apps/core/response.py

@@ -0,0 +1,160 @@
+"""
+Unified API response wrapper.
+
+Provides consistent response format across all API endpoints:
+{
+    "success": bool,
+    "data": {...} | [...] | null,
+    "errors": [...] | null,
+    "meta": {
+        "request_id": "uuid",
+        "pagination": {...}  // optional
+    }
+}
+"""
+
+from typing import Any
+
+from rest_framework import status
+from rest_framework.response import Response
+
+
+def api_response(
+    data: Any = None,
+    *,
+    status_code: int = status.HTTP_200_OK,
+    request_id: str | None = None,
+    pagination: dict[str, Any] | None = None,
+    headers: dict[str, str] | None = None,
+) -> Response:
+    """
+    Create a successful API response.
+
+    Args:
+        data: Response data (dict, list, or None)
+        status_code: HTTP status code (default 200)
+        request_id: Request tracking ID
+        pagination: Pagination metadata
+        headers: Additional response headers
+
+    Returns:
+        DRF Response with unified format
+    """
+    meta = {}
+    if request_id:
+        meta["request_id"] = request_id
+    if pagination:
+        meta["pagination"] = pagination
+
+    response_data = {
+        "success": True,
+        "data": data,
+        "errors": None,
+        "meta": meta if meta else None,
+    }
+
+    return Response(response_data, status=status_code, headers=headers)
+
+
+def api_error_response(
+    errors: list[dict[str, Any]],
+    *,
+    status_code: int = status.HTTP_400_BAD_REQUEST,
+    request_id: str | None = None,
+    headers: dict[str, str] | None = None,
+) -> Response:
+    """
+    Create an error API response.
+
+    Args:
+        errors: List of error dictionaries, each with 'code' and 'message'
+        status_code: HTTP status code (default 400)
+        request_id: Request tracking ID
+        headers: Additional response headers
+
+    Returns:
+        DRF Response with unified error format
+    """
+    meta = {}
+    if request_id:
+        meta["request_id"] = request_id
+
+    response_data = {
+        "success": False,
+        "data": None,
+        "errors": errors,
+        "meta": meta if meta else None,
+    }
+
+    return Response(response_data, status=status_code, headers=headers)
+
+
+def api_created_response(
+    data: Any = None,
+    *,
+    request_id: str | None = None,
+    headers: dict[str, str] | None = None,
+) -> Response:
+    """Shortcut for 201 Created response."""
+    return api_response(
+        data,
+        status_code=status.HTTP_201_CREATED,
+        request_id=request_id,
+        headers=headers,
+    )
+
+
+def api_no_content_response(
+    *,
+    request_id: str | None = None,
+    headers: dict[str, str] | None = None,
+) -> Response:
+    """Shortcut for 204 No Content response."""
+    meta = {}
+    if request_id:
+        meta["request_id"] = request_id
+
+    return Response(
+        {"success": True, "data": None, "errors": None, "meta": meta if meta else None},
+        status=status.HTTP_204_NO_CONTENT,
+        headers=headers,
+    )
+
+
+def api_paginated_response(
+    data: list[Any],
+    *,
+    page: int,
+    page_size: int,
+    total_count: int,
+    request_id: str | None = None,
+    headers: dict[str, str] | None = None,
+) -> Response:
+    """
+    Create a paginated API response.
+
+    Args:
+        data: List of items for current page
+        page: Current page number
+        page_size: Number of items per page
+        total_count: Total number of items
+        request_id: Request tracking ID
+        headers: Additional response headers
+    """
+    total_pages = (total_count + page_size - 1) // page_size if page_size > 0 else 0
+
+    pagination = {
+        "page": page,
+        "page_size": page_size,
+        "total_count": total_count,
+        "total_pages": total_pages,
+        "has_next": page < total_pages,
+        "has_previous": page > 1,
+    }
+
+    return api_response(
+        data,
+        request_id=request_id,
+        pagination=pagination,
+        headers=headers,
+    )