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

internal/handlers/source_management.go

@@ -109,6 +109,11 @@ type DeliveryView struct {
 	// the middle of Results. The page must show it, or the
 	// bound would hide history rather than fold it.
 	AttemptsOmitted int
+
+	// Paused is set while the delivery is retrying and its
+	// target's circuit breaker is turning deliveries away, and
+	// nil otherwise.
+	Paused *PausedView
 }
 
 // eventLogTarget is what the event log needs to know about
@@ -1181,7 +1186,7 @@ func (h *Handlers) eventLogViews(
 	}
 
 	for i := range rows {
-		result[i].Deliveries = newDeliveryViews(
+		result[i].Deliveries = h.newDeliveryViews(
 			eventDeliveries[i], targetMap, attempts,
 		)
 		result[i].ResubmitCount = resubmits[rows[i].ID]
@@ -1305,8 +1310,9 @@ func (h *Handlers) loadDeliveryResults(
 
 // newDeliveryViews projects deliveries for rendering,
 // resolving each one's target to its display-safe view and
-// each one's attempts through that target's redactor.
-func newDeliveryViews(
+// each one's attempts through that target's redactor. A
+// retrying delivery also reads its target's circuit breaker.
+func (h *Handlers) newDeliveryViews(
 	deliveries []database.Delivery,
 	targetMap map[string]eventLogTarget,
 	attempts map[string][]deliveryResultRow,
@@ -1329,6 +1335,10 @@ func newDeliveryViews(
 			AttemptCount:    len(rows),
 			AttemptsOmitted: omitted,
 		}
+
+		if deliveries[i].Status == database.DeliveryStatusRetrying {
+			views[i].Paused = h.pausedView(deliveries[i].TargetID)
+		}
 	}
 
 	return views