Implement isolated user-engine MVP

src/user_engine/errors.py

@@ -0,0 +1,23 @@
+"""Typed exceptions raised by user-engine services."""
+
+from __future__ import annotations
+
+
+class UserEngineError(Exception):
+    """Base class for user-engine failures."""
+
+
+class AuthorizationDenied(UserEngineError):
+    """Raised when the authorization port denies an operation."""
+
+
+class ConflictError(UserEngineError):
+    """Raised when a write would violate a uniqueness constraint."""
+
+
+class NotFoundError(UserEngineError):
+    """Raised when a requested domain object does not exist."""
+
+
+class ValidationError(UserEngineError):
+    """Raised when input fails domain validation."""