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/apps.py

@@ -0,0 +1,7 @@
+from django.apps import AppConfig
+
+
+class CoreConfig(AppConfig):
+    default_auto_field = "django.db.models.BigAutoField"
+    name = "apps.core"
+    verbose_name = "Core"