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

A target whose circuit breaker had tripped still showed as Active, and its deliveries sat at a bare "retrying" with no attempts. Its row on the webhook page now says deliveries are paused after repeated failures and until when the cooldown ends, adding that one waiting delivery is then sent to test the target; while half-open it says deliveries are held while one tests it, with no time. Waiting deliveries show "next try no earlier than" the later of the cooldown and their own backoff, with the date when not today. The engine gains one read of a breaker's state and remaining cooldown under one lock, and shares the backoff formula.

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