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

internal/delivery/target_http.go

@@ -234,7 +234,7 @@ func (c *httpCore) handleRetry(
 		database.DeliveryStatusRetrying,
 	)
 
-	backoff := calcBackoff(attemptNum)
+	backoff := Backoff(attemptNum)
 
 	retryTask := *task
 	retryTask.AttemptNum = attemptNum + 1
@@ -301,7 +301,7 @@ func (c *httpCore) remainingBackoff(
 		return 0
 	}
 
-	backoff := calcBackoff(attemptNum)
+	backoff := Backoff(attemptNum)
 	elapsed := time.Since(lastResult.CreatedAt)
 	remaining := backoff - elapsed
 
@@ -326,12 +326,14 @@ func (c *httpCore) backoffElapsed(
 		return true
 	}
 
-	backoff := calcBackoff(attemptNum)
+	backoff := Backoff(attemptNum)
 
 	return time.Since(lastResult.CreatedAt) >= backoff
 }
 
-func calcBackoff(attemptNum int) time.Duration {
+// Backoff is how long an http or slack target with retries waits after
+// a delivery's failed attempt attemptNum before trying it again.
+func Backoff(attemptNum int) time.Duration {
 	shift := max(attemptNum-1, 0)
 	shift = min(shift, maxBackoffShift)