refactor(auth): Use user_id as JWT sub and implement get_user_by_id

Switched from username to user_id as the primary identifier in JWT tokens to better support external authentication providers. Added get_user_by_id to HistoryManager and updated API dependencies and tests to reflect these changes.

tests/api/test_api_auth.py

@@ -92,10 +92,10 @@ def test_oidc_callback_success():
 def test_get_me_success():
     """Test getting current user with a valid token."""
     from ea_chatbot.api.utils import create_access_token
-    token = create_access_token(data={"sub": "test@example.com", "user_id": "123"})
+    token = create_access_token(data={"sub": "123"})
     
     with patch("ea_chatbot.api.dependencies.history_manager") as mock_hm:
-        mock_hm.get_user.return_value = User(id="123", username="test@example.com", display_name="Test")
+        mock_hm.get_user_by_id.return_value = User(id="123", username="test@example.com", display_name="Test")
         
         response = client.get(
             "/auth/me",