Add GitLab compare link to the summary table

[lesnik512]

Adds a Compare column after Pending in the summary table. It links to GitLab's compare view, running from production to the tag that will ship.

• From: the production ref if it was deployed from a tag, otherwise the production SHA (labelled with the first 8 characters).
• To: the newest tag among the pending rows. In a --jira report, release.tag instead.
• Empty: when there is no production deployment, no tag in range, or the service failed to collect.
• URL: {project_url}/-/compare/{from}...{to}, the base-first form GitLab's docs show (main...feature-branch). Ref names go through urllib.parse.quote, so / is kept and # becomes %23.

Why these choices

• Label. The link is labelled from...tag rather than being attached to the "N changes" count. The count runs to the default-branch head, and the link stops at the tag.
• Tag detection. GitLab's deployment object has no tag flag; the deploying job (deployable.tag) has one. It is now carried as EnvironmentState.tag, defaulting to false. Old reports still render and fall back to the SHA, so schema_version stays 2.

Not verified

• Whether GitLab resolves an unescaped / in a compare path. The tests only check the URL string.