perf: support Deep Purple app matrices

[tf-pablo]

Overview

Jira ticket: N/A

Allow monorepo callers of the existing frontend PR workflow to run Deep Purple once per affected app immediately after preview deployment. Single-app callers keep the current app-name behavior.

Changes

• add an optional deep-purple-apps JSON-array input
• expand the existing Deep Purple reusable-workflow job with a matrix when that input is provided
• preserve app-name as the fallback for existing callers

Testing

• YAML parses successfully locally
• Typeform/performance-analytics consumed this branch from draft PR #2279
• a dummy responses-analytics change produced exactly one Deep Purple matrix job
• Deep Purple started after Deploy Preview while Integration Tests were still running
• the complete consumer workflow passed in 10m22s; Deep Purple passed in 3m16s

Docs

• Yes! ✋ I have updated the documentation.

For contributions to the Typeform/.github repo

Note: Please do not use this repository for new internal shared workflows and actions. Use https://github.com/Typeform/.github-private instead!

• This PR only changes an existing workflow.
• This PR adds a new workflow that is needed in a public Typeform repository.

[tf-pablo]

Closing this draft; the branch is retained only for the cross-repository CI experiment.