🐛 fix model management module spelling

[ump45nose]

Summary

• rename the misspelled model_managment_app module to model_management_app
• update the config application import and all test mock targets
• align the related test filenames and English/Chinese documentation references

This is a mechanical rename only; application behavior is unchanged.

Fixes #3698

Smoke checks

• python3 -m compileall -q backend/apps/model_management_app.py backend/apps/config_app.py
• git diff --check
• verified the repository has no remaining model_managment references

The full pytest suite was not run because this checkout does not have the project test dependencies installed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!