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/delivery/export_test.go

@@ -212,6 +212,14 @@ func (e *Engine) ExportSetCircuitBreaker(
 	e.httpTarget.circuitBreakers.Store(targetID, cb)
 }
 
+// ExportSetSlackCircuitBreaker is ExportSetCircuitBreaker for the
+// slack target.
+func (e *Engine) ExportSetSlackCircuitBreaker(
+	targetID string, cb *CircuitBreaker,
+) {
+	e.slackTarget.circuitBreakers.Store(targetID, cb)
+}
+
 // ExportParseHTTPConfig exposes parseHTTPConfig.
 func (e *Engine) ExportParseHTTPConfig(
 	configJSON string,