Show a target paused by its circuit breaker (closes #385)

A target whose circuit breaker had tripped still showed as Active, and its deliveries sat at a bare "retrying" with no attempts. Its row on the webhook page now says deliveries are paused after repeated failures and until when the cooldown ends, adding that one waiting delivery is then sent to test the target; while half-open it says deliveries are held while one tests it, with no time. Waiting deliveries show "next try no earlier than" the later of the cooldown and their own backoff, with the date when not today. The engine gains one read of a breaker's state and remaining cooldown under one lock, and shares the backoff formula.

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