feat(payment): 补充订阅套餐币种字段的 SQL 迁移文件

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/migrations/177_add_subscription_plan_currency.sql

@@ -0,0 +1,4 @@
+-- Display-only ISO 4217 currency label for subscription plan prices; empty
+-- keeps existing plans rendering without any label.
+ALTER TABLE subscription_plans
+    ADD COLUMN IF NOT EXISTS currency VARCHAR(3) NOT NULL DEFAULT '';