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 "Max Retries" line, built by the same
code as an http target's. It shows no queue size: delivery never
reads one.

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:
@@ -119,10 +119,11 @@ 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 count. 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()
 	}
@@ -130,7 +131,7 @@ func slackConfigFields(configJSON string) []ConfigField {
 	return []ConfigField{{
 		Label: "Webhook URL",
 		Value: cfg.MaskedWebhookURL(),
-	}}
+	}, maxRetriesField(t)}
 }
 
 // httpConfigFields describes an HTTP target: its destination
@@ -170,21 +171,7 @@ func httpConfigFields(t *database.Target) []ConfigField {
 		})
 	}
 
-	return append(fields, retryFields(t)...)
-}
-
-// retryFields describes a target's retry settings, which live
-// on the target row rather than in its configuration blob.
-func retryFields(t *database.Target) []ConfigField {
-	retries := strconv.Itoa(t.MaxRetries)
-	if t.MaxRetries == 0 {
-		retries += " (fire-and-forget)"
-	}
-
-	fields := []ConfigField{{
-		Label: "Max Retries",
-		Value: retries,
-	}}
+	fields = append(fields, maxRetriesField(t))
 
 	if t.MaxQueueSize > 0 {
 		fields = append(fields, ConfigField{
@@ -196,6 +183,20 @@ func retryFields(t *database.Target) []ConfigField {
 	return fields
 }
 
+// maxRetriesField describes a target's retry count, which lives
+// on the target row rather than in its configuration blob.
+func maxRetriesField(t *database.Target) ConfigField {
+	retries := strconv.Itoa(t.MaxRetries)
+	if t.MaxRetries == 0 {
+		retries += " (fire-and-forget)"
+	}
+
+	return ConfigField{
+		Label: "Max Retries",
+		Value: retries,
+	}
+}
+
 // databaseConfigFields describes an archive target. Its
 // configuration is optional, and an absent or empty expiry
 // means the archive is kept forever. An expiry that is set