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/event_log.go

@@ -496,7 +496,7 @@ const deliveryIDChunkSize = 500
 //
 // Each response body is cut by SQLite rather than in Go, for
 // the reason deliveryResultColumns gives. How many attempts a
-// delivery has is the target's MaxRetries, which the
+// delivery has is bounded by the target's MaxAttempts, which the
 // authenticated operator sets; how many of them reach the page
 // is bounded again by maxRenderedAttempts.
 func (h *Handlers) loadDeliveryResults(