fix: use SQL migration for legacy purchase_subscription_url migration

Replace Go startup code with SQL migration 096 to migrate
purchase_subscription_url to custom_menu_items. Database migrations
run exactly once and are tracked by the migration system.

backend/internal/service/payment_service.go

@@ -150,10 +150,8 @@ func NewPaymentService(entClient *dbent.Client, registry *payment.Registry, load
 }
 
 // Start performs one-time startup tasks.
-func (s *PaymentService) Start(ctx context.Context) {
-	if err := s.configService.MigrateLegacyPurchaseURL(ctx); err != nil {
-		slog.Error("[payment] failed to migrate legacy purchase URL", "error", err)
-	}
+// Legacy purchase_subscription_url migration is handled by SQL migration 096.
+func (s *PaymentService) Start(_ context.Context) {
 }
 
 func (s *PaymentService) CreateOrder(ctx context.Context, req CreateOrderRequest) (*CreateOrderResponse, error) {

backend/migrations/096_migrate_purchase_subscription_to_custom_menu.sql

@@ -0,0 +1,70 @@
+-- 096_migrate_purchase_subscription_to_custom_menu.sql
+--
+-- Migrates the legacy purchase_subscription_url setting into custom_menu_items.
+-- After migration, purchase_subscription_enabled is set to "false" and
+-- purchase_subscription_url is cleared.
+--
+-- Idempotent: skips if custom_menu_items already contains
+-- "migrated_purchase_subscription".
+
+DO $$
+DECLARE
+    v_enabled  text;
+    v_url      text;
+    v_raw      text;
+    v_items    jsonb;
+    v_new_item jsonb;
+BEGIN
+    -- Read legacy settings
+    SELECT value INTO v_enabled
+      FROM settings WHERE key = 'purchase_subscription_enabled';
+    SELECT value INTO v_url
+      FROM settings WHERE key = 'purchase_subscription_url';
+
+    -- Skip if not enabled or URL is empty
+    IF COALESCE(v_enabled, '') <> 'true' OR COALESCE(TRIM(v_url), '') = '' THEN
+        RETURN;
+    END IF;
+
+    -- Read current custom_menu_items
+    SELECT value INTO v_raw
+      FROM settings WHERE key = 'custom_menu_items';
+
+    IF COALESCE(v_raw, '') = '' OR v_raw = 'null' THEN
+        v_items := '[]'::jsonb;
+    ELSE
+        v_items := v_raw::jsonb;
+    END IF;
+
+    -- Skip if already migrated (item with id "migrated_purchase_subscription" exists)
+    IF EXISTS (
+        SELECT 1 FROM jsonb_array_elements(v_items) elem
+         WHERE elem ->> 'id' = 'migrated_purchase_subscription'
+    ) THEN
+        RETURN;
+    END IF;
+
+    -- Build the new menu item
+    v_new_item := jsonb_build_object(
+        'id',         'migrated_purchase_subscription',
+        'label',      'Purchase',
+        'icon_svg',   '',
+        'url',        TRIM(v_url),
+        'visibility', 'user',
+        'sort_order', 100
+    );
+
+    -- Append to array
+    v_items := v_items || jsonb_build_array(v_new_item);
+
+    -- Upsert custom_menu_items
+    INSERT INTO settings (key, value)
+    VALUES ('custom_menu_items', v_items::text)
+    ON CONFLICT (key) DO UPDATE SET value = EXCLUDED.value;
+
+    -- Clear legacy settings
+    UPDATE settings SET value = 'false' WHERE key = 'purchase_subscription_enabled';
+    UPDATE settings SET value = ''      WHERE key = 'purchase_subscription_url';
+
+    RAISE NOTICE '[migration-096] Migrated purchase_subscription_url (%) to custom_menu_items', v_url;
+END $$;