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

templates/MetaLab-2026/static/js/studio-editor.js

@@ -190,6 +190,8 @@ async function handleSubmit(e) {
         const reprintSource = document.getElementById('reprintSource')?.value.trim() || ''
         const declaration = document.getElementById('declaration')?.value || ''
         const reprintProhibited = document.getElementById('reprintProhibited')?.checked || false
+        const categoryId = document.getElementById('categoryId')?.value || ''
+        const tags = document.getElementById('tagInput')?.value.trim() || ''
 
         const isEdit = !!postId
         const url = isEdit ? '/api/studio/posts/' + postId : '/api/studio/posts'
@@ -207,6 +209,8 @@ async function handleSubmit(e) {
                 reprint_source: reprintSource,
                 declaration,
                 reprint_prohibited: reprintProhibited,
+                category_id: categoryId,
+                tags,
             }),
         })
         const data = await resp.json()