fix(server): load PR uploads from orgs that enforce SAML SSO#88

What Changed

Carries pingdotgg/t3code#19191 sso-pr-media by cbangera2, adapted to the GitHub source-control driver's media fetch. When github.com answers a pull request upload with its SAML SSO sign-in page, the driver renders the upload through the markdown API, which accepts the SSO-authorized token and returns a signed private-user-images URL, then fetches that URL without the token. The media request now carries the checkout, so an upload whose URL names no repository is rendered with the checkout's own GitHub repository as context, including a project rooted at a bare clone. Signed URLs are cached for four of their five minutes so video seeks reuse them. Organizations without SSO make no extra requests.

Why

Images and videos in pull request descriptions from SSO-enforcing organizations show "Image unavailable" in the PR panel. Drop this layer at the refresh after #19191 sso-pr-media merges upstream.

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes