Handle hotfix upgrade messaging and clear stale update state after applying updates.
- UpdateController: when notifying admins for hotfixes, pass the current app version as the "to" version so notifications read e.g. "v1.1.3 has been updated" instead of showing a commit SHA.
- NotificationService: adjust system upgrade message to show a simple "has been updated" message when fromVersion equals toVersion (hotfix/file updates) and standard "upgraded from X to Y" otherwise; factor migration label formatting.
- UpdateService: clear cached update-check state after a successful update (add clearUpdateCache method) so the UI no longer shows a stale "update available" card. Keep latest_available_version unchanged to avoid view inconsistencies before migrations run.
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