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, then one is sent to test the target. Each of its
retrying deliveries shows as waiting in the event log and on the
event's page, with the earliest it can be tried next: the later of the
cooldown's end and the end of its own backoff, dated when not today
in UTC. 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

@@ -2366,6 +2366,20 @@ 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 earliest it can be tried
+next: the later of the cooldown's end and the end of its own backoff
+after its last attempt. It is only the earliest: when the cooldown ends,
+one of the target's waiting deliveries is sent to test it while the
+others wait at least one more cooldown, as the row also says. A time not
+on the current UTC day is shown with its date. 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