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. The column is NOT NULL and a check constraint refuses a value
below 1, so every target, whatever its type, is stored with at least
1. 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/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,
 	)
 }