Show a slack target's retry setting in the target list (closes #395)

The target list showed only the masked webhook URL for a slack
target, though its edit page saves a retry setting and delivery
honours it. It now shows the same retry fields, with the same
labels and wording, as an http target.

Model: opus-5-5

internal/delivery/target_config_view.go

@@ -97,7 +97,7 @@ func targetConfigFields(
 ) []ConfigField {
 	switch t.Type {
 	case database.TargetTypeSlack:
-		return slackConfigFields(t.Config)
+		return slackConfigFields(t)
 	case database.TargetTypeHTTP:
 		return httpConfigFields(t)
 	case database.TargetTypeDatabase:
@@ -119,18 +119,19 @@ func unavailableConfigFields() []ConfigField {
 	}}
 }
 
-// slackConfigFields describes a Slack target. Only the masked
-// webhook URL is shown; the full URL is the credential.
-func slackConfigFields(configJSON string) []ConfigField {
-	cfg, err := parseSlackConfig(configJSON)
+// slackConfigFields describes a Slack target: its masked
+// webhook URL and its retry settings. 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 {
 		return unavailableConfigFields()
 	}
 
-	return []ConfigField{{
+	return append([]ConfigField{{
 		Label: "Webhook URL",
 		Value: cfg.MaskedWebhookURL(),
-	}}
+	}}, retryFields(t)...)
 }
 
 // httpConfigFields describes an HTTP target: its destination