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/handlers/delivery_result_view.go

@@ -1,6 +1,8 @@
 package handlers
 
 import (
+	"time"
+
 	"sneak.berlin/go/webhooker/internal/delivery"
 )
 
@@ -24,8 +26,8 @@ const maxRenderedResponseBytes = 4096
 // bytes rather than characters, and they make SQLite do the
 // cut, so an oversized stored response never becomes a Go
 // string at all.
-const deliveryResultColumns = "delivery_id, attempt_num, success, " +
-	"status_code, error, duration, " +
+const deliveryResultColumns = "delivery_id, attempt_num, created_at, " +
+	"success, status_code, error, duration, " +
 	"substr(cast(response_body as blob), 1, ?) AS response_body, " +
 	"length(cast(response_body as blob)) AS response_bytes"
 
@@ -100,6 +102,7 @@ func (v DeliveryResultView) HasStatusCode() bool {
 type deliveryResultRow struct {
 	DeliveryID    string
 	AttemptNum    int
+	CreatedAt     time.Time
 	Success       bool
 	StatusCode    int
 	Error         string