feat: 分类系统 + 标签系统 — Phase 4 完成

分类：Model/Repo/Service/Controller + 管理后台树形页面 + 首页导航栏
标签：Model/Repo/Service/Controller + /tags/{slug} 落地页 + 写文章页输入
Post 加 CategoryID，Create/Update 支持分类和标签
SQL 迁移含 categories/tags/post_tags 表及预设未分类

internal/repository/category_repo.go

@@ -0,0 +1,63 @@
+package repository
+
+import (
+	"metazone.cc/mce/internal/model"
+	"metazone.cc/mce/internal/service"
+
+	"gorm.io/gorm"
+)
+
+// CategoryRepo 分类数据访问
+type CategoryRepo struct {
+	db *gorm.DB
+}
+
+// NewCategoryRepo 构造函数
+func NewCategoryRepo(db *gorm.DB) *CategoryRepo {
+	return &CategoryRepo{db: db}
+}
+
+// WithTx 基于事务创建新实例
+func (r *CategoryRepo) WithTx(tx *gorm.DB) service.CategoryStore {
+	return &CategoryRepo{db: tx}
+}
+
+// Create 创建分类
+func (r *CategoryRepo) Create(c *model.Category) error {
+	return r.db.Create(c).Error
+}
+
+// Update 更新分类
+func (r *CategoryRepo) Update(c *model.Category) error {
+	return r.db.Save(c).Error
+}
+
+// Delete 删除分类
+func (r *CategoryRepo) Delete(id uint) error {
+	return r.db.Delete(&model.Category{}, id).Error
+}
+
+// FindByID 按 ID 查询
+func (r *CategoryRepo) FindByID(id uint) (*model.Category, error) {
+	var c model.Category
+	err := r.db.First(&c, id).Error
+	if err != nil {
+		return nil, err
+	}
+	return &c, nil
+}
+
+// ListAll 查询全部分类（按 sort 排序）
+func (r *CategoryRepo) ListAll() ([]model.Category, error) {
+	var list []model.Category
+	err := r.db.Order("sort ASC, id ASC").Find(&list).Error
+	return list, err
+}
+
+// GetLeaves 查询所有叶子分类（有子分类的父节点不返回）
+func (r *CategoryRepo) GetLeaves() ([]model.Category, error) {
+	var leaves []model.Category
+	err := r.db.Where("id NOT IN (SELECT DISTINCT parent_id FROM categories WHERE parent_id IS NOT NULL)").
+		Order("sort ASC, id ASC").Find(&leaves).Error
+	return leaves, err
+}

internal/repository/tag_repo.go

@@ -0,0 +1,97 @@
+package repository
+
+import (
+	"metazone.cc/mce/internal/model"
+	"metazone.cc/mce/internal/service"
+
+	"gorm.io/gorm"
+)
+
+// TagRepo 标签数据访问
+type TagRepo struct {
+	db *gorm.DB
+}
+
+// NewTagRepo 构造函数
+func NewTagRepo(db *gorm.DB) *TagRepo {
+	return &TagRepo{db: db}
+}
+
+// WithTx 基于事务创建新实例
+func (r *TagRepo) WithTx(tx *gorm.DB) service.TagStore {
+	return &TagRepo{db: tx}
+}
+
+// FindOrCreate 查找或创建标签（按名字）
+func (r *TagRepo) FindOrCreate(name, slug string) (*model.Tag, error) {
+	var tag model.Tag
+	err := r.db.Where("slug = ?", slug).First(&tag).Error
+	if err == nil {
+		return &tag, nil
+	}
+	if err != gorm.ErrRecordNotFound {
+		return nil, err
+	}
+	tag = model.Tag{Name: name, Slug: slug}
+	if err := r.db.Create(&tag).Error; err != nil {
+		return nil, err
+	}
+	return &tag, nil
+}
+
+// FindBySlug 按 slug 查询
+func (r *TagRepo) FindBySlug(slug string) (*model.Tag, error) {
+	var tag model.Tag
+	err := r.db.Where("slug = ?", slug).First(&tag).Error
+	if err != nil {
+		return nil, err
+	}
+	return &tag, nil
+}
+
+// SetPostTags 设置文章的标签（删除旧的，插入新的）
+func (r *TagRepo) SetPostTags(postID uint, tagIDs []uint) error {
+	return r.db.Transaction(func(tx *gorm.DB) error {
+		if err := tx.Where("post_id = ?", postID).Delete(&model.PostTag{}).Error; err != nil {
+			return err
+		}
+		for _, tagID := range tagIDs {
+			if err := tx.Create(&model.PostTag{PostID: postID, TagID: tagID}).Error; err != nil {
+				return err
+			}
+		}
+		return nil
+	})
+}
+
+// GetPostTags 获取文章关联的标签
+func (r *TagRepo) GetPostTags(postID uint) ([]model.Tag, error) {
+	var tags []model.Tag
+	err := r.db.Table("tags").
+		Joins("JOIN post_tags ON post_tags.tag_id = tags.id").
+		Where("post_tags.post_id = ?", postID).
+		Find(&tags).Error
+	return tags, err
+}
+
+// GetPostsByTag 按标签查询文章（公开+已发布）
+func (r *TagRepo) GetPostsByTag(tagID uint, offset, limit int) ([]model.Post, int64, error) {
+	var total int64
+	query := r.db.Table("posts").
+		Joins("JOIN post_tags ON post_tags.post_id = posts.id").
+		Joins("LEFT JOIN users ON users.id = posts.user_id").
+		Where("post_tags.tag_id = ?", tagID).
+		Where("posts.status = ?", model.PostStatusApproved).
+		Where("posts.visibility = ?", model.VisibilityPublic).
+		Where("posts.deleted_at IS NULL").
+		Select("posts.*, users.username as author_name")
+
+	if err := query.Count(&total).Error; err != nil {
+		return nil, 0, err
+	}
+
+	var posts []model.Post
+	err := query.Order("CASE WHEN posts.pin_type = 'global' THEN 0 ELSE 1 END ASC, posts.pinned_at DESC NULLS LAST, posts.created_at DESC").
+		Offset(offset).Limit(limit).Find(&posts).Error
+	return posts, total, err
+}