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_create_query_test.go

@@ -196,7 +196,7 @@ func TestHandleTargetCreate_QueryStringCannotSupplyNameOrType(
 
 	w, _ := postTargetCreate(
 		t, env, webhook.ID,
-		"name=leaky&type=slack&max_retries=9&expiry=30d&url="+
+		"name=leaky&type=slack&max_attempts=9&expiry=30d&url="+
 			url.QueryEscape(targetSecretURL),
 		url.Values{},
 	)