Replace the old testflight-release workflow with a single
`testflight.yml` Gitea Actions workflow and rework the fastlane `beta`
lane: match-based app-store signing into a disposable CI keychain,
XcodeGen project generation, version/build-number bumping, and upload
to TestFlight. Pin Ruby to 3.1.7 for the runner.
Squashes the iterative CI signing debugging history into one commit.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
