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/controller/admin/category_controller.go

@@ -0,0 +1,100 @@
+package admin
+
+import (
+	"net/http"
+	"strconv"
+
+	"metazone.cc/mce/internal/common"
+	"metazone.cc/mce/internal/model"
+
+	"github.com/gin-gonic/gin"
+)
+
+// CategoryController 分类管理控制器
+type CategoryController struct {
+	svc categoryUseCase
+}
+
+type categoryUseCase interface {
+	Create(c *model.Category) error
+	Update(c *model.Category) error
+	Delete(id uint) error
+	Tree() ([]model.Category, error)
+}
+
+// NewCategoryController 构造函数
+func NewCategoryController(svc categoryUseCase) *CategoryController {
+	return &CategoryController{svc: svc}
+}
+
+// Page 分类管理页面
+func (cc *CategoryController) Page(c *gin.Context) {
+	tree, _ := cc.svc.Tree()
+	c.HTML(http.StatusOK, "admin/categories/index.html", common.BuildAdminPageData(c, gin.H{
+		"Title":    "分类管理",
+		"ExtraCSS": "/admin/static/css/categories.css",
+		"ExtraJS":  "/admin/static/js/categories.js",
+		"Tree":     tree,
+	}))
+}
+
+// List 分类列表 API
+func (cc *CategoryController) List(c *gin.Context) {
+	tree, err := cc.svc.Tree()
+	if err != nil {
+		common.Error(c, http.StatusInternalServerError, "查询失败")
+		return
+	}
+	if tree == nil {
+		tree = []model.Category{}
+	}
+	common.Ok(c, tree)
+}
+
+// Create 创建分类
+func (cc *CategoryController) Create(c *gin.Context) {
+	var req model.Category
+	if err := c.ShouldBindJSON(&req); err != nil {
+		common.Error(c, http.StatusBadRequest, "参数错误")
+		return
+	}
+	if err := cc.svc.Create(&req); err != nil {
+		common.Error(c, http.StatusInternalServerError, "创建失败")
+		return
+	}
+	common.OkMessage(c, "分类已创建")
+}
+
+// Update 更新分类
+func (cc *CategoryController) Update(c *gin.Context) {
+	id, err := strconv.ParseUint(c.Param("id"), 10, 64)
+	if err != nil {
+		common.Error(c, http.StatusBadRequest, "无效的 ID")
+		return
+	}
+	var req model.Category
+	if err := c.ShouldBindJSON(&req); err != nil {
+		common.Error(c, http.StatusBadRequest, "参数错误")
+		return
+	}
+	req.ID = uint(id)
+	if err := cc.svc.Update(&req); err != nil {
+		common.Error(c, http.StatusInternalServerError, "更新失败")
+		return
+	}
+	common.OkMessage(c, "分类已更新")
+}
+
+// Delete 删除分类
+func (cc *CategoryController) Delete(c *gin.Context) {
+	id, err := strconv.ParseUint(c.Param("id"), 10, 64)
+	if err != nil {
+		common.Error(c, http.StatusBadRequest, "无效的 ID")
+		return
+	}
+	if err := cc.svc.Delete(uint(id)); err != nil {
+		common.Error(c, http.StatusInternalServerError, "删除失败")
+		return
+	}
+	common.OkMessage(c, "分类已删除")
+}