Module name typo: `model_managment_app.py` should be `model_management_app.py`

Currently:

• backend/apps/model_managment_app.py
• test/backend/app/test_model_managment_app.py
• Referenced from backend/apps/config_app.py:16:

  from apps.model_managment_app import router as model_manager_router
  

• And mentioned in doc/docs/{en,zh}/backend/overview.md:25, doc/docs/{en,zh}/getting-started/software-architecture.md:35, doc/docs/{en,zh}/testing/backend.md:22-23.

managment is a misspelling of management. Every other module in backend/apps/ and every service/database peer use the correctly-spelled form, e.g.:

backend/apps/user_management_app.py
backend/services/model_management_service.py
backend/services/user_management_service.py
test/backend/services/test_model_management_service.py

So this file is the lone outlier — even its own service-layer counterpart got the spelling right. It's a small thing, but module-name typos compound: every git grep, every import, every doc reference now has to remember which subsystem mis-spelled it.

Fix

git mv backend/apps/model_managment_app.py backend/apps/model_management_app.py
git mv test/backend/app/test_model_managment_app.py test/backend/app/test_model_management_app.py

…then fix the import in backend/apps/config_app.py:16 and the four doc references.

Category: F (docs/code mismatch — and code/code mismatch with the rest of the module taxonomy). Severity: Low (cosmetic), but easy to ship the fix.

Start with backend/apps/model_managment_app.py, test/backend/app/test_model_managment_app.py, backend/apps/config_app.py:16, and the four documentation references listed in the issue. Rename the module and test, update every reference, then run the renamed backend app test and verify no managment references remain.