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/post_repo.go

@@ -36,7 +36,7 @@ func (r *PostRepo) FindByIDWithAuthor(id uint) (*model.Post, error) {
 	var post model.Post
 	err := r.db.Table("posts").
 		Select("posts.*, users.username as author_name").
-		Joins("LEFT JOIN users ON users.uid = posts.user_id").
+		Joins("LEFT JOIN users ON users.id = posts.user_id").
 		Where("posts.id = ?", id).
 		First(&post).Error
 	if err != nil {
@@ -49,7 +49,7 @@ func (r *PostRepo) FindByIDWithAuthor(id uint) (*model.Post, error) {
 func (r *PostRepo) buildQuery(keyword, status string) *gorm.DB {
 	query := r.db.Table("posts").
 		Select("posts.*, users.username as author_name").
-		Joins("LEFT JOIN users ON users.uid = posts.user_id").
+		Joins("LEFT JOIN users ON users.id = posts.user_id").
 		Where("posts.deleted_at IS NULL")
 
 	if keyword != "" {