refactor: 拆分 middleware/router 为单一职责文件，新增站点设置管理页面

middleware:
- 拆分 auth.go → auth.go + auth_token.go + auth_parser.go + auth_admin.go
- 拆分 csrf.go → csrf.go + csrf_token.go
- 拆分 ratelimit.go → ratelimit.go + ratelimit_core.go + ratelimit_cleanup.go
- 修复 import 未使用/缺失问题

router:
- 拆分 router.go → admin.go + api.go + frontend.go + deps_core.go + deps_extra.go

feat(admin): 站点设置管理页面
- 新增 SSR 页面 /admin/site-settings（仅 Owner）
- 审核开关 Toggle 即时保存
- 通用设置展示
- 侧边栏新增站点设置入口
- 注册 UpdateBoolSetting/GetBoolSetting API 路由

templates/admin/static/css/site-settings.css

@@ -0,0 +1,83 @@
+/* =====================================================
+   MetaLab 管理面板 - 站点设置
+   单一职责：设置页面布局、表单分组、开关组件
+   ===================================================== */
+
+/* --- 设置区块 --- */
+.settings-section {
+    background: var(--bg-card);
+    border-radius: var(--radius);
+    box-shadow: var(--shadow);
+    margin-bottom: 20px;
+    overflow: hidden;
+}
+.section-title {
+    font-size: 15px; font-weight: 600;
+    color: var(--text);
+    padding: 16px 20px;
+    border-bottom: 1px solid var(--border);
+    background: #fafafa;
+}
+.section-body { padding: 4px 0; }
+
+/* --- 设置条目 --- */
+.setting-item {
+    display: flex; align-items: center; justify-content: space-between;
+    padding: 14px 20px;
+    border-bottom: 1px solid #f5f5f5;
+    gap: 20px;
+}
+.setting-item:last-child { border-bottom: none; }
+.setting-info { flex: 1; min-width: 0; }
+.setting-label {
+    font-size: 13px; font-weight: 500;
+    color: var(--text); display: block;
+}
+.setting-desc {
+    font-size: 12px; color: var(--text-muted);
+    margin-top: 2px;
+}
+.setting-value {
+    font-size: 12px; color: var(--text-muted);
+    margin-top: 2px; font-family: monospace;
+    background: #f5f6fa; padding: 2px 6px; border-radius: 3px;
+    display: inline-block;
+    word-break: break-all;
+}
+
+/* --- 空状态 --- */
+.settings-empty {
+    text-align: center; padding: 28px 20px;
+    color: var(--text-muted); font-size: 13px;
+}
+
+/* --- Toggle 开关 --- */
+.toggle-switch {
+    position: relative; display: inline-block;
+    width: 44px; height: 24px; flex-shrink: 0;
+}
+.toggle-switch input { opacity: 0; width: 0; height: 0; }
+.toggle-slider {
+    position: absolute; cursor: pointer;
+    top: 0; left: 0; right: 0; bottom: 0;
+    background: #ccc;
+    border-radius: 24px;
+    transition: background 0.2s;
+}
+.toggle-slider::before {
+    position: absolute; content: "";
+    height: 18px; width: 18px;
+    left: 3px; bottom: 3px;
+    background: #fff;
+    border-radius: 50%;
+    transition: transform 0.2s;
+}
+.toggle-switch input:checked + .toggle-slider {
+    background: var(--primary);
+}
+.toggle-switch input:checked + .toggle-slider::before {
+    transform: translateX(20px);
+}
+.toggle-switch input:disabled + .toggle-slider {
+    opacity: 0.4; cursor: not-allowed;
+}