feat: implement repository and unit-of-work patterns for service layer operations

services/exceptions.py

@@ -0,0 +1,9 @@
+"""Domain-level exceptions for service and repository layers."""
+
+
+class EntityNotFoundError(Exception):
+    """Raised when a requested entity cannot be located."""
+
+
+class EntityConflictError(Exception):
+    """Raised when attempting to create or update an entity that violates uniqueness."""