feat: add nightly CI run for quickstart stack test with Slack alerts

[marythought]

🤖:

Summary

The quickstart docker-compose stack test (test-stack in docker-compose-test.yml) only runs on push/PR to paths under static/quickstart/, tests/, and docs/getting-started/. This means upstream breaking changes (like opentdf/platform#3563) can silently break the quickstart for users without any notification.

Proposed changes

1. Add a schedule trigger to run the stack test nightly
2. Add a Slack notification step (on failure only) to #alerts-opentdf

Branch with the workflow changes is ready: feat/quickstart-nightly-test

Prerequisite

A SLACK_WEBHOOK_URL repo secret needs to be added to opentdf/docs, pointing to an Incoming Webhook for #alerts-opentdf. Either:

• Ask in #ask-ops-nonprod for a webhook
• Or create one via https://api.slack.com/apps if you have Slack app admin access