refactor: BaseModel 主键列名统一 uid→id

- BaseModel 移除 gorm column:uid 和 json:uid 标签，统一为 id
- 更新 8 个 repository 文件中 28 处 raw SQL 列引用 (uid→id)
- 更新 3 个前端 JS 文件中 14 处 API 响应字段引用 (uid→id)
- 添加数据库迁移 SQL 脚本 (docs/migrations/001_uid_to_id.sql)

docs/migrations/001_uid_to_id.sql

@@ -0,0 +1,27 @@
+-- Migration: 重命名 BaseModel 使用表的主键列 uid → id
+-- 涉及表: users, audit_submissions, notifications, user_checkins, user_tasks
+-- 执行前请备份数据库
+
+BEGIN;
+
+-- users 表主键及序列重命名
+ALTER TABLE users RENAME COLUMN uid TO id;
+ALTER SEQUENCE IF EXISTS users_uid_seq RENAME TO users_id_seq;
+
+-- audit_submissions 表主键及序列重命名
+ALTER TABLE audit_submissions RENAME COLUMN uid TO id;
+ALTER SEQUENCE IF EXISTS audit_submissions_uid_seq RENAME TO audit_submissions_id_seq;
+
+-- notifications 表主键及序列重命名
+ALTER TABLE notifications RENAME COLUMN uid TO id;
+ALTER SEQUENCE IF EXISTS notifications_uid_seq RENAME TO notifications_id_seq;
+
+-- user_checkins 表主键及序列重命名
+ALTER TABLE user_checkins RENAME COLUMN uid TO id;
+ALTER SEQUENCE IF EXISTS user_checkins_uid_seq RENAME TO user_checkins_id_seq;
+
+-- user_tasks 表主键及序列重命名
+ALTER TABLE user_tasks RENAME COLUMN uid TO id;
+ALTER SEQUENCE IF EXISTS user_tasks_uid_seq RENAME TO user_tasks_id_seq;
+
+COMMIT;