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/test_workflow_history.py

@@ -25,6 +25,11 @@ def test_full_history_workflow(history_manager):
     assert user is not None
     assert user.display_name == "E2E User"
     
+    # 1.1 Verify get_user_by_id
+    fetched_user = history_manager.get_user_by_id(user.id)
+    assert fetched_user is not None
+    assert fetched_user.username == email
+    
     # 2. Create Conversation
     conv = history_manager.create_conversation(user.id, "nj", "Test Analytics")
     assert conv.id is not None