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_service.go

@@ -3,6 +3,7 @@ package service
 import (
 	"errors"
 	"fmt"
+	"strconv"
 	"time"
 
 	"metazone.cc/mce/internal/common"
@@ -68,7 +69,7 @@ func (s *PostService) findPostWithAuthor(id uint) (*model.Post, error) {
 // Create 创建帖子
 // body 为 Vditor 输出的 Markdown，后端直接存储不做 HTML 消毒
 // MD 是纯文本，无 XSS 注入风险；前端渲染时由 Vditor 转 HTML
-func (s *PostService) Create(userID uint, title, body string, visibility, postType, reprintSource, declaration string, reprintProhibited bool) (*model.Post, error) {
+func (s *PostService) Create(userID uint, title, body string, visibility, postType, reprintSource, declaration string, reprintProhibited bool, categoryID string) (*model.Post, error) {
 	status := model.PostStatusApproved
 	if s.auditEnabled() {
 		status = model.PostStatusDraft
@@ -87,6 +88,11 @@ func (s *PostService) Create(userID uint, title, body string, visibility, postTy
 		Declaration:       declaration,
 		ReprintProhibited: reprintProhibited,
 	}
+	// 解析分类 ID
+	if cid, err := strconv.ParseUint(categoryID, 10, 64); err == nil && cid > 0 {
+		id := uint(cid)
+		post.CategoryID = &id
+	}
 	if err := s.repo.Create(post); err != nil {
 		return nil, err
 	}