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/export_test.go

@@ -37,10 +37,10 @@ const MaxRenderedResponseBytesForTest = maxRenderedResponseBytes
 // per-delivery attempt ceiling to the handlers_test package.
 const MaxRenderedAttemptsForTest = maxRenderedAttempts
 
-// MaxTargetRetriesForTest exposes the target max_retries ceiling to
+// MaxTargetAttemptsForTest exposes the target max_attempts ceiling to
 // the handlers_test package, so the tests assert against the constant
 // the handlers enforce rather than a number copied beside it.
-const MaxTargetRetriesForTest = maxTargetRetries
+const MaxTargetAttemptsForTest = maxTargetAttempts
 
 // EventDBLeftMsgForTest and SidecarLeftMsgForTest expose the two
 // messages the webhook delete handler logs when a file of the event