Issue #3593895: operator test-notification button for dormancy delivery

On the Vault subjects page, a 'Send a test notification to me' button dispatches a sample pdv.dormancy warned event for the operator's own account (context flagged source=operator_test, no vault touched, no warn_count change), so they can confirm the notification pipeline - PDV Mail's email, and any ECA model - works before a real dormant vault triggers it. Shown whether or not dormancy is enabled.

Closes #3593895