fix(数据库): 补充默认分组种子迁移

确保安装时至少存在一个默认分组

backend/migrations/008_seed_default_group.sql

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