Lower WAL balance threshold to 2 and fix build

[ducnmm]

Summary

• Lower WAL balance warning threshold from 5 WAL to 2 WAL before posting alerts (using MIST conversion), for both default and user namespace checks.
• Fix CI regression in set-metadata step by matching set_metadata_batch returning Result<usize, AppError> instead of unit, preventing compile failure in clippy/tests/E2E.

Test status

• Not run locally in this environment (CI will validate).