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

While an http or slack target's circuit breaker turns its deliveries
away, the target's row on the webhook page says its deliveries are
paused and until when, in UTC and from now. Each of its retrying
deliveries shows as waiting, with the reason and the same time, in the
event log and on the event's page, instead of a bare "retrying".

The engine gains one read, CooldownRemaining(targetID), over the
breakers it already keeps; the handlers reach it through a one-method
interface wired like Archives.

Model: opus-5-5

README.md

@@ -2331,6 +2331,14 @@ 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 turning its deliveries away, the target's row
+on the webhook page says its deliveries are paused and until when, in UTC
+and as a time from now, and each of its `retrying` deliveries shows as
+waiting until then in the event log and on the event's page. That time is
+when the cooldown ends; while a half-open breaker's probe delivery is in
+flight, it is one whole cooldown from now, the delay the breaker gives the
+other deliveries.
+
 ### Metrics
 
 `/metrics` serves one Prometheus registry behind basic auth (see