feat: 实现内容系统 MVC（Post CRUD + 审核流转）

按 content-system.md 设计文档，实现帖子内容系统的最小可行 MVC。

新增：
- Model: Post 模型，含 5 种状态（draft/pending/approved/rejected/locked）
- Repository: post_repo.go，分页查询（前台 approved 列表 + 后台全状态筛选）
- Service: post_service.go，核心业务逻辑（状态流转、Markdown 渲染 Goldmark）
- Controller: post_controller + admin_post_controller，SSR 页面 6 个 + API 13 个
- Template: 列表/详情/编辑器/404 + 管理员列表，各配 CSS/JS
- 路由: /posts, /posts/:id, /posts/new, /posts/:id/edit, REST API, Admin API
- gomod: + github.com/yuin/goldmark v1.8.2

修改：
- Main: AutoMigrate Post 表
- Router: deps 注册 PostService/Controller，路由注册
- Nav: 新增“帖子”导航链接
- Admin Sidebar: 新增“内容管理”入口

设计：
- 审核开关复用 audit.enabled，开启时帖子为 draft，关闭后直接 approved
- 仅 approved 公开可见，作者/admin 可预览非公开状态帖子
- 严格分层 ISP 接口: Controller → postUseCase, Service → postStore, Repo → *PostRepo
- 状态保护: pending/locked 禁止编辑，rejected 编辑后自动重置为 draft

templates/admin/static/css/posts.css

@@ -0,0 +1,131 @@
+/* ========== 管理后台帖子列表 ========== */
+
+.admin-filter-bar {
+    margin-bottom: 20px;
+}
+
+.filter-form {
+    display: flex;
+    gap: 12px;
+    align-items: center;
+}
+
+.filter-input {
+    flex: 1;
+    max-width: 300px;
+    padding: 8px 12px;
+    border: 1px solid #d1d5db;
+    border-radius: 8px;
+    font-size: 14px;
+}
+
+.filter-select {
+    padding: 8px 12px;
+    border: 1px solid #d1d5db;
+    border-radius: 8px;
+    font-size: 14px;
+    background: #fff;
+}
+
+.admin-table {
+    width: 100%;
+    border-collapse: collapse;
+    background: #fff;
+    border-radius: 10px;
+    overflow: hidden;
+    box-shadow: 0 1px 3px rgba(0,0,0,0.06);
+}
+
+.admin-table th {
+    background: #f9fafb;
+    padding: 12px 16px;
+    text-align: left;
+    font-size: 13px;
+    font-weight: 600;
+    color: #6b7280;
+    border-bottom: 1px solid #e5e7eb;
+}
+
+.admin-table td {
+    padding: 12px 16px;
+    font-size: 14px;
+    border-bottom: 1px solid #f3f4f6;
+    color: #374151;
+}
+
+.admin-table .empty {
+    text-align: center;
+    color: #9ca3af;
+    padding: 40px 0;
+}
+
+.post-link {
+    color: #6366f1;
+    text-decoration: none;
+    font-weight: 500;
+}
+
+.post-link:hover { text-decoration: underline; }
+
+.status-badge {
+    display: inline-block;
+    padding: 2px 10px;
+    border-radius: 12px;
+    font-size: 12px;
+    font-weight: 500;
+}
+
+.status-draft { background: #f3f4f6; color: #6b7280; }
+.status-pending { background: #fef3c7; color: #d97706; }
+.status-approved { background: #d1fae5; color: #059669; }
+.status-rejected { background: #fee2e2; color: #dc2626; }
+.status-locked { background: #fee2e2; color: #b91c1c; }
+
+.actions-cell { min-width: 180px; }
+
+.action-btns {
+    display: flex;
+    gap: 6px;
+    flex-wrap: wrap;
+}
+
+.btn-sm {
+    padding: 4px 12px;
+    font-size: 12px;
+    border-radius: 6px;
+    border: none;
+    cursor: pointer;
+    font-weight: 500;
+    transition: opacity 0.2s;
+}
+
+.btn-sm:hover { opacity: 0.85; }
+
+.btn-success { background: #10b981; color: #fff; }
+.btn-warning { background: #f59e0b; color: #fff; }
+.btn-danger { background: #ef4444; color: #fff; }
+
+/* 分页 */
+.pagination {
+    display: flex;
+    justify-content: center;
+    align-items: center;
+    gap: 16px;
+    margin-top: 24px;
+}
+
+.page-link {
+    padding: 8px 14px;
+    border: 1px solid #d1d5db;
+    border-radius: 8px;
+    color: #374151;
+    text-decoration: none;
+    font-size: 13px;
+}
+
+.page-link:hover { background: #f3f4f6; }
+
+.page-info {
+    font-size: 13px;
+    color: #6b7280;
+}

templates/admin/static/js/posts.js

@@ -0,0 +1,61 @@
+(function() {
+    function getCSRFToken() {
+        var meta = document.querySelector('meta[name="csrf-token"]');
+        return meta ? meta.getAttribute('content') : '';
+    }
+
+    function api(url, method, body) {
+        return fetch(url, {
+            method: method,
+            headers: {
+                'Content-Type': 'application/json',
+                'X-CSRF-Token': getCSRFToken()
+            },
+            body: body ? JSON.stringify(body) : undefined
+        }).then(function(r) { return r.json(); });
+    }
+
+    function refreshPage() { window.location.reload(); }
+
+    function handleClick(sel, actionFn) {
+        var btns = document.querySelectorAll(sel);
+        btns.forEach(function(btn) {
+            btn.addEventListener('click', function() {
+                var id = btn.getAttribute('data-id');
+                if (confirm('确定执行此操作吗？')) {
+                    actionFn(id);
+                }
+            });
+        });
+    }
+
+    // 审核通过
+    handleClick('.post-approve', function(id) {
+        api('/api/admin/posts/' + id + '/approve', 'POST')
+            .then(function(d) { if (d.success) refreshPage(); else alert(d.message); })
+            .catch(function() { alert('操作失败'); });
+    });
+
+    // 退回
+    handleClick('.post-reject', function(id) {
+        var reason = prompt('请输入退回理由（必填）：');
+        if (!reason || !reason.trim()) return;
+        api('/api/admin/posts/' + id + '/reject', 'POST', { reason: reason.trim() })
+            .then(function(d) { if (d.success) refreshPage(); else alert(d.message); })
+            .catch(function() { alert('操作失败'); });
+    });
+
+    // 锁定
+    handleClick('.post-lock', function(id) {
+        api('/api/admin/posts/' + id + '/lock', 'POST')
+            .then(function(d) { if (d.success) refreshPage(); else alert(d.message); })
+            .catch(function() { alert('操作失败'); });
+    });
+
+    // 解锁
+    handleClick('.post-unlock', function(id) {
+        api('/api/admin/posts/' + id + '/unlock', 'POST')
+            .then(function(d) { if (d.success) refreshPage(); else alert(d.message); })
+            .catch(function() { alert('操作失败'); });
+    });
+})();