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_config_view.go

@@ -120,8 +120,8 @@ func unavailableConfigFields() []ConfigField {
 }
 
 // slackConfigFields describes a Slack target: its masked
-// webhook URL and its retry count. Only the masked URL is
-// shown; the full URL is the credential.
+// webhook URL and its number of delivery attempts. Only the
+// masked URL is shown; the full URL is the credential.
 func slackConfigFields(t *database.Target) []ConfigField {
 	cfg, err := parseSlackConfig(t.Config)
 	if err != nil {
@@ -131,7 +131,7 @@ func slackConfigFields(t *database.Target) []ConfigField {
 	return []ConfigField{{
 		Label: "Webhook URL",
 		Value: cfg.MaskedWebhookURL(),
-	}, maxRetriesField(t)}
+	}, maxAttemptsField(t)}
 }
 
 // httpConfigFields describes an HTTP target: its destination
@@ -178,23 +178,18 @@ func httpConfigFields(t *database.Target) []ConfigField {
 		})
 	}
 
-	fields = append(fields, maxRetriesField(t))
+	fields = append(fields, maxAttemptsField(t))
 
 	return fields
 }
 
-// maxRetriesField describes a target's retry count, which lives
-// on the target row rather than in its configuration blob. A
-// stored 0 makes a single attempt, so it is shown as 1.
-func maxRetriesField(t *database.Target) ConfigField {
-	attempts := strconv.Itoa(t.MaxRetries)
-	if t.MaxRetries == 0 {
-		attempts = "1 (fire-and-forget: no retries, no circuit breaker)"
-	}
-
+// maxAttemptsField describes a target's number of delivery
+// attempts, which lives on the target row rather than in its
+// configuration blob.
+func maxAttemptsField(t *database.Target) ConfigField {
 	return ConfigField{
 		Label: "Delivery attempts",
-		Value: attempts,
+		Value: strconv.Itoa(t.MaxAttempts),
 	}
 }