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)

internal/repository/comment_mention_repo.go

@@ -35,7 +35,7 @@ func (r *CommentRepo) SearchUsersByLevel(keyword string, minLevel int, limit int
 	threshold := model.LevelThresholds[minLevel]
 
 	err := r.db.Table("users").
-		Select("uid, username, exp").
+		Select("id, username, exp").
 		Where("username LIKE ? AND exp >= ? AND deleted_at IS NULL", "%"+keyword+"%", threshold).
 		Order("exp DESC").
 		Limit(limit).
@@ -59,7 +59,7 @@ func (r *CommentRepo) SearchUsersByLevel(keyword string, minLevel int, limit int
 // GetUserUIDByID 通过 uint ID 获取 user 的 UID 字符串
 func (r *CommentRepo) GetUserUIDByID(userID uint) (string, error) {
 	var uid uint
-	err := r.db.Table("users").Select("uid").Where("uid = ?", userID).Scan(&uid).Error
+	err := r.db.Table("users").Select("id").Where("id = ?", userID).Scan(&uid).Error
 	if err != nil {
 		return "", err
 	}
@@ -69,7 +69,7 @@ func (r *CommentRepo) GetUserUIDByID(userID uint) (string, error) {
 // GetUsernameByID 通过 userID 获取用户名（用于通知内容）
 func (r *CommentRepo) GetUsernameByID(userID uint) (string, error) {
 	var username string
-	err := r.db.Table("users").Select("username").Where("uid = ?", userID).Scan(&username).Error
+	err := r.db.Table("users").Select("username").Where("id = ?", userID).Scan(&username).Error
 	return username, err
 }
 
@@ -112,7 +112,7 @@ func (r *CommentRepo) AggregateCommentsByAuthor(userID uint, since string) ([]se
 func (r *CommentRepo) ListAllComments(keyword string, showDeleted bool, offset, limit int) ([]model.AdminCommentRow, int64, error) {
 	var total int64
 	q := r.db.Table("comments").
-		Joins("LEFT JOIN users ON users.uid = comments.user_id").
+		Joins("LEFT JOIN users ON users.id = comments.user_id").
 		Joins("LEFT JOIN posts ON posts.id = comments.post_id")
 
 	if !showDeleted {