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/target_slack.go

@@ -30,9 +30,8 @@ type SlackTargetConfig struct {
 // slackTarget delivers events to Slack incoming webhooks. It
 // formats the event into a Slack message and posts it as
 // JSON. It shares the retry core with the HTTP target: a
-// MaxRetries of 0 stays single-attempt fire-and-forget
-// (preserving existing Slack targets), while a positive
-// MaxRetries adds backoff and circuit breaking.
+// MaxAttempts of 1 is a single attempt, while a higher one
+// adds backoff and circuit breaking.
 type slackTarget struct {
 	*httpCore
 
@@ -83,7 +82,7 @@ func (t *slackTarget) Deliver(
 
 	t.deliver(
 		ctx, webhookDB, d, task, sched,
-		d.Target.MaxRetries, attempt,
+		d.Target.MaxAttempts, attempt,
 	)
 }