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/inflight_test.go

@@ -229,7 +229,7 @@ func TestOwnershipIsReleasedAfterDelivery(t *testing.T) {
 
 	iCreateTarget(t, s.MainDB, targetID,
 		s.WebhookID, "released",
-		database.TargetTypeLog, "", 0,
+		database.TargetTypeLog, "", 1,
 	)
 
 	event := iSeedEvent(