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/delivery/event_db_isolation_test.go

@@ -152,7 +152,7 @@ func TestEventDBHoldsNoTargetRowsOnFailedDelivery(t *testing.T) {
 	)
 	task := iTask(
 		d, event, s.WebhookID, targetID,
-		"leaky-target", cfg, 0, 1, &body,
+		"leaky-target", cfg, 1, 1, &body,
 	)
 
 	s.Engine.ExportProcessNewTask(context.TODO(), &task)