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/008_seed_default_group.sql

@@ -1,4 +1,4 @@
--- Seed a default group for fresh installs.
-INSERT INTO groups (name, description, created_at, updated_at)
-SELECT 'default', 'Default group', NOW(), NOW()
-WHERE NOT EXISTS (SELECT 1 FROM groups);
+-- Seed a default group for fresh installs.
+INSERT INTO groups (name, description, created_at, updated_at)
+SELECT 'default', 'Default group', NOW(), NOW()
+WHERE NOT EXISTS (SELECT 1 FROM groups);