refactor: 删除 users 表中的冗余飞书字段

- 删除 feishu_user_id, feishu_open_id, tenant_key 字段
- JWT token 改用 feishu_app_id 代替 feishu_user_id
- 删除废弃的 FeishuService::sendApprovalMessage() 方法
- 更新 User 模型移除废弃字段和 scope 方法

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/database/migrations/013_remove_users_feishu_fields.sql

@@ -0,0 +1,32 @@
+-- X-Server 删除 users 表中的冗余飞书字段
+-- 这些字段的数据已迁移至 user_feishu_apps 表
+
+USE xserver;
+
+-- =====================================
+-- 删除冗余的飞书字段
+-- =====================================
+
+-- 删除 feishu_user_id
+ALTER TABLE users DROP COLUMN feishu_user_id;
+
+-- 删除 feishu_open_id
+ALTER TABLE users DROP COLUMN feishu_open_id;
+
+-- 删除 tenant_key
+ALTER TABLE users DROP COLUMN tenant_key;
+
+-- =====================================
+-- 验证结果
+-- =====================================
+
+SELECT "=== users 表结构验证 ===" as "";
+DESCRIBE users;
+
+SELECT "=== user_feishu_apps 表统计 ===" as "";
+SELECT
+    company_id,
+    COUNT(DISTINCT user_id) as users_count,
+    COUNT(*) as bindings_count
+FROM user_feishu_apps
+GROUP BY company_id;