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_services.py

@@ -0,0 +1,103 @@
+"""Tests for core services"""
+
+from apps.core.exceptions import NotFoundError
+from apps.core.services import BaseService
+from django.contrib.auth import get_user_model
+from django.test import TestCase
+
+User = get_user_model()
+
+
+class UserTestService(BaseService[User]):
+    """Test service using User model"""
+
+    model = User
+
+
+class BaseServiceTest(TestCase):
+    """Tests for BaseService"""
+
+    def setUp(self):
+        self.user = User.objects.create_user(
+            username="testuser",
+            email="test@example.com",
+            password="testpass123",
+        )
+
+    def test_get_by_id_success(self):
+        """Test get_by_id returns entity"""
+        result = UserTestService.get_by_id(self.user.pk)
+        self.assertEqual(result.pk, self.user.pk)
+        self.assertEqual(result.email, self.user.email)
+
+    def test_get_by_id_not_found(self):
+        """Test get_by_id raises NotFoundError for non-existent ID"""
+        with self.assertRaises(NotFoundError) as context:
+            UserTestService.get_by_id(99999)
+
+        self.assertEqual(context.exception.code, "not_found")
+        self.assertIn("User", context.exception.message)
+
+    def test_get_by_id_or_none_found(self):
+        """Test get_by_id_or_none returns entity when found"""
+        result = UserTestService.get_by_id_or_none(self.user.pk)
+        self.assertEqual(result.pk, self.user.pk)
+
+    def test_get_by_id_or_none_not_found(self):
+        """Test get_by_id_or_none returns None when not found"""
+        result = UserTestService.get_by_id_or_none(99999)
+        self.assertIsNone(result)
+
+    def test_get_all(self):
+        """Test get_all returns queryset"""
+        User.objects.create_user(
+            username="testuser2",
+            email="test2@example.com",
+            password="testpass123",
+        )
+
+        result = UserTestService.get_all()
+        self.assertEqual(result.count(), 2)
+
+    def test_filter(self):
+        """Test filter returns filtered queryset"""
+        result = UserTestService.filter(email="test@example.com")
+        self.assertEqual(result.count(), 1)
+        self.assertEqual(result.first().email, "test@example.com")
+
+    def test_exists_true(self):
+        """Test exists returns True when entity exists"""
+        result = UserTestService.exists(email="test@example.com")
+        self.assertTrue(result)
+
+    def test_exists_false(self):
+        """Test exists returns False when entity does not exist"""
+        result = UserTestService.exists(email="nonexistent@example.com")
+        self.assertFalse(result)
+
+    def test_count_all(self):
+        """Test count returns total count"""
+        result = UserTestService.count()
+        self.assertEqual(result, 1)
+
+    def test_count_filtered(self):
+        """Test count with filter"""
+        result = UserTestService.count(email="test@example.com")
+        self.assertEqual(result, 1)
+
+        result = UserTestService.count(email="nonexistent@example.com")
+        self.assertEqual(result, 0)
+
+    def test_update(self):
+        """Test update modifies entity fields"""
+        UserTestService.update(self.user, username="updated_username")
+
+        self.user.refresh_from_db()
+        self.assertEqual(self.user.username, "updated_username")
+
+    def test_delete(self):
+        """Test delete removes entity"""
+        user_pk = self.user.pk
+        UserTestService.delete(self.user)
+
+        self.assertFalse(User.objects.filter(pk=user_pk).exists())