Docs: Align Server .env.example with README

Hey @manishkumar8312

### Description
There is a discrepancy between the server environment variables listed in the main `README.md` and the variables present in the `server/sample.env` file.

The `README.md` correctly lists the following Cloudinary variables as required:
- `CLOUDINARY_CLOUD_NAME`
- `CLOUDINARY_API_KEY`
- `CLOUDINARY_API_SECRET`

However, these variables are missing from `server/sample.env`. This can cause confusion for new contributors during setup.

### Proposed Solution
Update `server/sample.env` to include the missing Cloudinary variables with placeholder values. This will ensure the example file is a complete and accurate template.

Compare the Cloudinary variables listed in README.md with server/sample.env. Add the three missing variables with placeholder values, then verify that the sample environment file matches the README's required server variables.

Written by the indexing model from the issue text.