feat(): migrate to auth identity

src/template_project/application/auth_identity/errors.py

@@ -0,0 +1,17 @@
+from typing import TYPE_CHECKING, override
+
+from template_project.application.auth_identity.entity import AuthMethod
+from template_project.application.common.errors import ApplicationError, to_error
+
+if TYPE_CHECKING:
+    pass
+
+
+@to_error
+class UserAlreadyExistsError(ApplicationError):
+    identifier: str
+    auth_method: AuthMethod
+
+    @override
+    def __str__(self) -> str:
+        return f"User with identifier={self.identifier!r} and auth method={self.auth_method.value} already exists"