Add PR preview workflow for testing changes before merge

[Copilot]

Enables testing pull requests by automatically building and publishing preview artifacts.

Changes

• New workflow .github/workflows/pr-preview.yml:

  • Triggers on PR open/update events
  • Injects secrets (APIFY_TOKEN, GOOGLE_TOKEN) into static files
  • Uploads artifact with explicit file list to exclude sensitive content (.git, .github, etc.)
  • Posts/updates PR comment with download link and testing instructions
  • 7-day artifact retention

• Documentation updates in README.md explaining the preview workflow

Usage

When a PR is opened:

1. Workflow runs automatically
2. Bot comments with artifact download link
3. Download, extract, serve locally to test
4. Comment updates on subsequent pushes

No changes to existing deployment workflow.

Original prompt

add support for publishing from pull requests for testing purposes

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