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

tests/apps/user/__init__.py

@@ -0,0 +1,10 @@
+"""
+Tests for user application
+
+This package contains all tests for the user app including:
+- Model tests
+- Serializer tests
+- Service tests
+- View tests
+- Factory classes for test data generation
+"""