BTHLABS-61: Service layer refactoring

A journey to fix `ValidationError` in Pocket imports turned service
layer refactoring :D

services/packages/soa/hotpocket_soa/constants.py

@@ -0,0 +1,11 @@
+# -*- coding: utf-8 -*-
+from __future__ import annotations
+
+import enum
+
+
+class BackendServiceErrorCode(enum.Enum):
+    INTERNAL = -32000
+    NOT_FOUND = -32001
+    ACCESS_DENIED = -32002
+    INVALID = -32003