Enable Deploy Previews for PRs

[keiranjprice101]

Description of the issue

introduce deploy previews** for pull requests across our services (fia-api, frontend, and data viewer). This will make it easier to test changes in isolation before merging, and allow reviewers to interact with a running version of the branch.

Desired Workflow (possiblity, could be different)

1. PR Opened
   • The branch is automatically deployed to a preview environment.
   • Accessible via a URL like:

     <branch-name>.dev.reduce.isis.cclrc.ac.uk
     

   • nice to have: comment link to new deploy in PR
2. PR Merged or Closed
   • The preview deployment is automatically removed/cleaned up.

Considerations

• We use GitOps, so this may involve:
  • A GitHub Action that updates the GitOps repo with the preview deployment manifests.
  • Triggering image builds (for API, frontend, data viewer) as part of the PR pipeline.
  • Cleaning up GitOps state when the PR is closed.
• Need to decide on naming conventions and limits (e.g., handling long branch names, invalid characters).