feat(firmware): dismiss stale failed upgrade notice

[gnacho]

A failed firmware upgrade attempt (e.g. "agent not connected" from an old incident) stayed on the page forever with no way to clear it, even when the router was healthy again and on the target version.

• New endpoint DELETE /api/firmware-upgrades/{routerId}/failure: removes the latest finished (failed/done) upgrade record for the router; an in-progress upgrade is never touched.
• Dismiss button on the failed-notice banner (shown to admins).
• Fixed a latent store bug: LatestUpgrade/GetUpgradeByID failed to scan NULL error/backup_path, so in-progress upgrades were silently missing from the list.

Closes #519