GitHub's github.ref_name returns "421/merge" for PR merge refs,
creating invalid Docker tags like "pr-421/merge". Docker tags
cannot contain forward slashes.
Changed to use github.event.pull_request.number which returns
just the PR number (e.g., "421") for valid tags like "pr-421".
Also added comprehensive unit tests for backup_service.go to
meet the 85% coverage threshold.
Fixes CI/CD failure in PR #421.