feat: implement ActivityService and TaskService with business logic for activities and tasks

app/services/__init__.py

@@ -1,9 +1,26 @@
 """Business logic services."""
+from .activity_service import (  # noqa: F401
+    ActivityForbiddenError,
+    ActivityListFilters,
+    ActivityService,
+    ActivityServiceError,
+    ActivityValidationError,
+)
+from .auth_service import AuthService  # noqa: F401
 from .organization_service import (  # noqa: F401
     OrganizationAccessDeniedError,
     OrganizationContext,
     OrganizationContextMissingError,
     OrganizationService,
 )
-from .user_service import UserService  # noqa: F401
-from .auth_service import AuthService  # noqa: F401
+from .task_service import (  # noqa: F401
+    TaskDueDateError,
+    TaskForbiddenError,
+    TaskListFilters,
+    TaskNotFoundError,
+    TaskOrganizationError,
+    TaskService,
+    TaskServiceError,
+    TaskUpdateData,
+)
+from .user_service import UserService  # noqa: F401