feat: 品牌重命名 Sub2API -> TianShuAPI

- 前端: 所有界面显示、i18n 文本、组件中的品牌名称
- 后端: 服务层、设置默认值、邮件模板、安装向导
- 数据库: 迁移脚本注释
- 保持功能完全一致，仅更改品牌名称

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

backend/migrations/006_fix_invalid_subscription_expires_at.sql

@@ -1,10 +1,10 @@
--- Fix legacy subscription records with invalid expires_at (year > 2099).
-DO $$
-BEGIN
-  IF to_regclass('public.user_subscriptions') IS NOT NULL THEN
-    UPDATE user_subscriptions
-    SET expires_at = TIMESTAMPTZ '2099-12-31 23:59:59+00'
-    WHERE expires_at > TIMESTAMPTZ '2099-12-31 23:59:59+00';
-  END IF;
-END $$;
-
+-- Fix legacy subscription records with invalid expires_at (year > 2099).
+DO $$
+BEGIN
+  IF to_regclass('public.user_subscriptions') IS NOT NULL THEN
+    UPDATE user_subscriptions
+    SET expires_at = TIMESTAMPTZ '2099-12-31 23:59:59+00'
+    WHERE expires_at > TIMESTAMPTZ '2099-12-31 23:59:59+00';
+  END IF;
+END $$;
+