feat(explorer): inactive-dataset email links to the triage queue

### Description

When a dataset crosses the inactivity threshold from #465, email the payer with a link to `/console/services/[operator]#stale`. Email uses the wallet-to-email binding from the notifications work.

### Expected behavior

One email per dataset per threshold crossing. Muted (Keep) datasets never email. Link deep-links to the queue with the dataset highlighted.

### Additional context

Parent: #460. Depends on #465 and on the email alerts backend. The POC's `AlertPreferencesCard` is not the implementation; alert categories extend the existing Email Alerts page.

Merges alone: backend job plus one email template; the console link target exists once #465 merges and otherwise resolves to the service page.

Start by reviewing #465, the email alerts backend, and the existing Email Alerts page; treat AlertPreferencesCard as a POC rather than the implementation. Trace the wallet-to-email binding and the `/console/services/[operator]#stale` target. Done means one email per dataset per threshold crossing, no email for muted datasets, and a highlighted queue link.

Written by the indexing model from the issue text.