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/core/test_exceptions.py

@@ -0,0 +1,152 @@
+"""Tests for core exceptions and exception handler"""
+
+from apps.core.exceptions import (
+    AuthenticationError,
+    BadRequestError,
+    BaseAPIException,
+    BusinessLogicError,
+    ConflictError,
+    DuplicateError,
+    InternalError,
+    InvalidStateError,
+    NotFoundError,
+    PermissionDeniedError,
+    QuotaExceededError,
+    RateLimitError,
+    ServiceUnavailableError,
+    ValidationError,
+)
+from django.test import TestCase
+
+
+class BaseAPIExceptionTest(TestCase):
+    """Tests for BaseAPIException"""
+
+    def test_default_values(self):
+        """Test exception with default values"""
+        exc = BaseAPIException()
+        self.assertEqual(exc.message, "An error occurred")
+        self.assertEqual(exc.code, "error")
+        self.assertEqual(exc.status_code, 400)
+        self.assertIsNone(exc.details)
+
+    def test_custom_values(self):
+        """Test exception with custom values"""
+        exc = BaseAPIException(
+            message="Custom message",
+            code="custom_code",
+            details={"field": "value"},
+        )
+        self.assertEqual(exc.message, "Custom message")
+        self.assertEqual(exc.code, "custom_code")
+        self.assertEqual(exc.details, {"field": "value"})
+
+    def test_to_dict(self):
+        """Test conversion to dictionary"""
+        exc = BaseAPIException(
+            message="Test message",
+            code="test_code",
+            details={"key": "value"},
+        )
+        result = exc.to_dict()
+
+        self.assertEqual(result["message"], "Test message")
+        self.assertEqual(result["code"], "test_code")
+        self.assertEqual(result["details"], {"key": "value"})
+
+    def test_to_dict_without_details(self):
+        """Test to_dict without details"""
+        exc = BaseAPIException(message="Test")
+        result = exc.to_dict()
+
+        self.assertNotIn("details", result)
+
+
+class ClientErrorExceptionsTest(TestCase):
+    """Tests for client error exceptions (4xx)"""
+
+    def test_validation_error(self):
+        """Test ValidationError defaults"""
+        exc = ValidationError()
+        self.assertEqual(exc.status_code, 400)
+        self.assertEqual(exc.code, "validation_error")
+
+    def test_bad_request_error(self):
+        """Test BadRequestError defaults"""
+        exc = BadRequestError()
+        self.assertEqual(exc.status_code, 400)
+        self.assertEqual(exc.code, "bad_request")
+
+    def test_authentication_error(self):
+        """Test AuthenticationError defaults"""
+        exc = AuthenticationError()
+        self.assertEqual(exc.status_code, 401)
+        self.assertEqual(exc.code, "authentication_error")
+
+    def test_permission_denied_error(self):
+        """Test PermissionDeniedError defaults"""
+        exc = PermissionDeniedError()
+        self.assertEqual(exc.status_code, 403)
+        self.assertEqual(exc.code, "permission_denied")
+
+    def test_not_found_error(self):
+        """Test NotFoundError defaults"""
+        exc = NotFoundError()
+        self.assertEqual(exc.status_code, 404)
+        self.assertEqual(exc.code, "not_found")
+
+    def test_conflict_error(self):
+        """Test ConflictError defaults"""
+        exc = ConflictError()
+        self.assertEqual(exc.status_code, 409)
+        self.assertEqual(exc.code, "conflict")
+
+    def test_rate_limit_error(self):
+        """Test RateLimitError defaults"""
+        exc = RateLimitError()
+        self.assertEqual(exc.status_code, 429)
+        self.assertEqual(exc.code, "rate_limit_exceeded")
+
+
+class ServerErrorExceptionsTest(TestCase):
+    """Tests for server error exceptions (5xx)"""
+
+    def test_internal_error(self):
+        """Test InternalError defaults"""
+        exc = InternalError()
+        self.assertEqual(exc.status_code, 500)
+        self.assertEqual(exc.code, "internal_error")
+
+    def test_service_unavailable_error(self):
+        """Test ServiceUnavailableError defaults"""
+        exc = ServiceUnavailableError()
+        self.assertEqual(exc.status_code, 503)
+        self.assertEqual(exc.code, "service_unavailable")
+
+
+class BusinessLogicExceptionsTest(TestCase):
+    """Tests for business logic exceptions"""
+
+    def test_business_logic_error(self):
+        """Test BusinessLogicError defaults"""
+        exc = BusinessLogicError()
+        self.assertEqual(exc.status_code, 400)
+        self.assertEqual(exc.code, "business_error")
+
+    def test_invalid_state_error(self):
+        """Test InvalidStateError defaults"""
+        exc = InvalidStateError()
+        self.assertEqual(exc.status_code, 400)
+        self.assertEqual(exc.code, "invalid_state")
+
+    def test_duplicate_error(self):
+        """Test DuplicateError defaults"""
+        exc = DuplicateError()
+        self.assertEqual(exc.status_code, 409)
+        self.assertEqual(exc.code, "duplicate")
+
+    def test_quota_exceeded_error(self):
+        """Test QuotaExceededError defaults"""
+        exc = QuotaExceededError()
+        self.assertEqual(exc.status_code, 400)
+        self.assertEqual(exc.code, "quota_exceeded")