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/service/post_audit_service.go

@@ -1,12 +1,14 @@
 package service
 
 import (
+	"strconv"
+
 	"metazone.cc/mce/internal/common"
 	"metazone.cc/mce/internal/model"
 )
 
 // Update 编辑帖子（权限在 controller 层检查）
-func (s *PostService) Update(postID uint, title, body string, visibility, postType, reprintSource, declaration string, reprintProhibited bool) error {
+func (s *PostService) Update(postID uint, title, body string, visibility, postType, reprintSource, declaration string, reprintProhibited bool, categoryID string) error {
 	post, err := s.findPost(postID)
 	if err != nil {
 		return err
@@ -22,6 +24,13 @@ func (s *PostService) Update(postID uint, title, body string, visibility, postTy
 	post.ReprintSource = reprintSource
 	post.Declaration = declaration
 	post.ReprintProhibited = reprintProhibited
+	// 分类
+	if cid, err := strconv.ParseUint(categoryID, 10, 64); err == nil && cid > 0 {
+		id := uint(cid)
+		post.CategoryID = &id
+	} else {
+		post.CategoryID = nil
+	}
 
 	// 已通过帖子：编辑内容保存为待审修订，不影响当前展示
 	if post.Status == model.PostStatusApproved {