Configure testing deployment slot for new repository

[ashggan]

Summary

Set up a testing deployment slot in Azure App Service to deploy
from this repository (SpringACT/digital-safe) for isolated testing.

Tasks

• Create testing slot in Azure App Service (digital-safe-nodejs-app)
• Create workflow: testing_digital-safe-nodejs-app.yml
• Add GitHub secret: AZUREAPPSERVICE_PUBLISHPROFILE_TESTING
• Configure Node.js 24 runtime in testing slot
• Test deployment end-to-end

Related Issues

• Set up Continuous Integration #224: Set up Continuous Integration
• Upgrade to Node.js 24 by April 2026 #285: Upgrade to Node.js 24

Notes

This enables blue-green deployment and safe testing of the
cleaned-up codebase before production migration.