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_timestamp_test.go

@@ -176,7 +176,7 @@ func tsTask(
 		TargetName:   target.Name,
 		TargetType:   database.TargetTypeSlack,
 		TargetConfig: target.Config,
-		MaxRetries:   0,
+		MaxAttempts:  1,
 		Method:       event.Method,
 		Headers:      event.Headers,
 		ContentType:  event.ContentType,