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; a blank field on the add target form gives 1, and on the
edit form keeps the stored value. 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