Ramsundar - Backend fixes for Inventory Types page functionality

[Ram-blip]

Description

If items are added under Tools and Reusables, they are already added to their respective sections correctly. So, no changes were needed for those first two points. Only the third point was an issue, and this PR fixes that issue.

Related PRS (if any):

To test this backend PR you need to checkout the latest development frontend branch.

Main changes explained:

• The Unit of Measurement section previously relied on a local JSON file for persistence, which caused add/delete operations to fail on the dev website.
• Migrated “unit of measurement” storage from the old JSON-file approach to a MongoDB model, The backend now reads, adds, and deletes units through the database.

How to test:

1. check into current branch
2. do npm install and ... to run this PR locally
3. Clear site data/cache
4. log as admin user
5. Go to "http://localhost:5173/bmdashboard/inventorytypes"
6. First check: Go to unit of measurement, add any measurement, it will get added. Also, the delete will also work. Error page won't come.)

Screenshots or videos of changes:

Loom video After:
https://www.loom.com/share/a2295a396bac498788fc2bfd5b54e2fb

Note:

This PR is a replacement for #2131 and includes the updated implementation.