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/delivery/circuit_breaker.go

@@ -102,6 +102,20 @@ func (cb *CircuitBreaker) CooldownRemaining() time.Duration {
 	return remaining
 }
 
+// StateAndCooldown returns the circuit state and, while the circuit is
+// open, what is left of the cooldown, or zero once that has passed.
+// Both are read under one lock, so they always agree.
+func (cb *CircuitBreaker) StateAndCooldown() (CircuitState, time.Duration) {
+	cb.mu.Lock()
+	defer cb.mu.Unlock()
+
+	if cb.state != CircuitOpen {
+		return cb.state, 0
+	}
+
+	return cb.state, max(cb.cooldown-time.Since(cb.lastFailure), 0)
+}
+
 // RecordSuccess records a successful delivery and resets
 // the circuit breaker to closed state.
 func (cb *CircuitBreaker) RecordSuccess() {