Show a target paused by its circuit breaker (closes #385)

While an http or slack target's circuit breaker is open, the target's
row on the webhook page says its deliveries are paused until the
cooldown ends, in UTC and from now. Each of its retrying deliveries
shows as waiting in the event log and on the event's page, until the
later of the cooldown's end and the end of its own backoff. While the
breaker is half-open, the row says deliveries are held while one
delivery tests the target, with no time, and deliveries keep their
plain status.

The engine gains one read, StateAndCooldown(targetID), taking a
breaker's state and remaining cooldown under one lock; the handlers
reach it through a one-method interface wired like Archives.

Model: opus-5-5

README.md

@@ -2367,6 +2367,16 @@ just delayed until the target is healthy again. A delivery already in
 `retrying` keeps that status without another database write each time
 the breaker turns it away.
 
+While a target's breaker is open, the target's row on the webhook page
+says its deliveries are paused until the cooldown ends, in UTC and as a
+time from now. Each of its `retrying` deliveries shows as waiting in the
+event log and on the event's page, with the time it will be tried next:
+the later of the cooldown's end and the end of its own backoff after its
+last attempt. While the breaker is half-open, the row says instead that
+deliveries are held while one delivery tests whether the target has
+recovered, with no time, and the target's deliveries show their plain
+status, since any of them may be the one being sent.
+
 ### Metrics
 
 `/metrics` serves one Prometheus registry behind basic auth (see