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/target_edit.go

@@ -82,7 +82,7 @@ func (h *Handlers) HandleTargetEdit() http.HandlerFunc {
 			Headers:      cfg.Headers,
 			Timeout:      cfg.Timeout,
 			ForwardQuery: cfg.ForwardQuery,
-			MaxRetries:   strconv.Itoa(target.MaxRetries),
+			MaxAttempts:  strconv.Itoa(target.MaxAttempts),
 			Expiry:       cfg.Expiry,
 			Rotation:     cfg.Rotation,
 		}