Store a target's number of delivery attempts, at least one (closes #540)

Target.MaxRetries (max_retries) becomes Target.MaxAttempts
(max_attempts), and the form field, parser and ceiling are renamed to
match. 1 is a single attempt with no circuit breaker; above 1 retries
behind the breaker up to that many attempts. Both target forms refuse
0 and less and default a blank field to 1. The target page shows the
plain number, and the README's two paragraphs on 0 against 1 become
one sentence.

Model: opus-5-5

internal/handlers/webhook.go

@@ -442,7 +442,7 @@ func buildDeliveryTasks(
 			TargetName:   targets[i].Name,
 			TargetType:   targets[i].Type,
 			TargetConfig: targets[i].Config,
-			MaxRetries:   targets[i].MaxRetries,
+			MaxAttempts:  targets[i].MaxAttempts,
 			Method:       event.Method,
 			RawQuery:     event.RawQuery,
 			Headers:      event.Headers,