fix: 修复付费会员累加逻辑和迁移误判问题

- 修复 confirm_payment_order 累加逻辑：GREATEST(COALESCE(paid_expires_at, NOW()), NOW()) + days
- 永久会员改为 2099-12-31 而非 NULL，避免与未付费用户的 NULL 混淆
- 禁用 003_set_all_users_paid.sql 迁移（将所有已有用户设为永久付费）
- 后端部署后已回滚 Dev 数据库中 73 个被误设为付费的用户

migrations/003_set_all_users_paid.sql

@@ -1 +1,6 @@
-UPDATE users SET is_paid = true WHERE is_paid = false;
+-- ATTENTION: This migration was originally `UPDATE users SET is_paid = true WHERE is_paid = false`
+-- which incorrectly made ALL existing users permanent paid members (paid_expires_at = NULL → permanent).
+-- The accumulation logic now handles membership correctly in confirm_payment_order.
+-- DO NOT re-enable without understanding the full consequences.
+-- This file is kept empty as a placeholder to prevent migration numbering gaps.
+SELECT 1 WHERE 1 = 1;  -- no-op