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, in UTC and from now. Each of its retrying deliveries
shows as waiting in the event log and on the event's page, until the
later of the cooldown's end and the end of its own backoff. 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/server/routes_test.go

@@ -62,6 +62,17 @@ func (e *noopArchives) Rename(_, _, _ string) error {
 	return nil
 }
 
+// noopCircuitBreakers satisfies handlers.New's
+// delivery.CircuitBreakers dependency with no target's deliveries
+// paused.
+type noopCircuitBreakers struct{}
+
+func (b *noopCircuitBreakers) StateAndCooldown(
+	string,
+) (delivery.CircuitState, time.Duration) {
+	return delivery.CircuitClosed, 0
+}
+
 // testEnv is the real router from routes.go plus the collaborators
 // tests need to seed users and forge sessions.
 type testEnv struct {
@@ -126,6 +137,9 @@ func newTestEnvWithConfig(
 			session.New,
 			func() delivery.Notifier { return &noopNotifier{} },
 			func() delivery.Archives { return &noopArchives{} },
+			func() delivery.CircuitBreakers {
+				return &noopCircuitBreakers{}
+			},
 			metrics.NewRegistry,
 			metrics.New,
 			middleware.New,