Extend `/health` to return health of schedulers like last email sent and when the outbox poller last run.

Sometimes a fatal error can stop a scheduled job, like the email outbox poller.

We can extend the `/health` API to return the health of schedulers like have "last email sent" and "when the outbox poller last run".

Consider event notifications and entitlement pushes here too.

Start at the /health API entry point and trace the scheduler implementations it reports. Inspect the email outbox poller, event notifications, and entitlement pushes to determine which health timestamps are available. Done means /health exposes the requested scheduler health details, including the last email sent and the last outbox poller run.

Written by the indexing model from the issue text.