refactor: CSP nonce替代unsafe-inline, HSTS始终启用, router/api拆分, 管理后台+用户设置页拆分, DB健康降级, 时区配置, UID统一解析, CI接入

审计修复:
- CSP: unsafe-inline移除, nonce替代; unsafe-eval保留供Vditor使用
- HSTS: 始终启用(不再依赖release模式)
- Controller Exp: common.GetGinExp包装, 不再直接读context
- UID解析: common.ParseUIDParam统一, follow/admin controller改用

重构:
- router/api.go(198行)拆为7个域文件: auth/settings/posts/comments/reactions/social/studio
- 管理后台站点设置: 单页拆为4个子页(brand/security/registration/content)+子菜单
- 前台用户设置页: 1201行拆为6个独立模板+6个独立JS文件

新增:
- DB健康检测中间件(middleware/db_health.go): 5s ping+降级页面
- 时区配置: config.yaml server.timezone→time.Local初始化
- .gitea/workflows/ci.yml: strict模式CI流水线

templates/admin/html/site-settings/security.html

@@ -0,0 +1,47 @@
+{{template "admin/layout/base.html" .}}
+        <div class="page-header">
+            <h1>站点设置</h1>
+            <p class="page-desc">安全策略与维护模式</p>
+        </div>
+
+        {{template "admin/site-settings/_menu.html" .}}
+
+        <!-- 安全设置 -->
+        <div class="settings-section">
+            <div class="section-title">安全设置</div>
+            <div class="section-body">
+                <div class="setting-item">
+                    <div class="setting-info">
+                        <span class="setting-label">密码强度</span>
+                        <span class="setting-desc">注册和修改密码时的最低强度要求</span>
+                    </div>
+                    <div class="setting-input-wrap">
+                        <select id="setting-password-strength" class="setting-select" data-key="password.strength">
+                            <option value="low">低 — 仅要求 8 位</option>
+                            <option value="medium" selected>中 — 字母+数字/符号（两类）</option>
+                            <option value="high">高 — 大小写+数字+符号（至少三类）+ 禁止连续字符</option>
+                            <option value="very_high">极高 — 四类全有 + 禁止连续字符</option>
+                        </select>
+                        <button class="btn btn-sm btn-primary btn-save-select" data-key="password.strength" data-input="setting-password-strength">保存</button>
+                    </div>
+                </div>
+            </div>
+        </div>
+
+        <!-- 维护设置 -->
+        <div class="settings-section">
+            <div class="section-title">维护设置</div>
+            <div class="section-body">
+                <div class="setting-item">
+                    <div class="setting-info">
+                        <span class="setting-label">启用维护模式</span>
+                        <span class="setting-desc">开启后仅站长可登录和访问，其他用户将看到维护提示</span>
+                    </div>
+                    <label class="toggle-switch">
+                        <input type="checkbox" id="maintenance-enabled" data-key="maintenance.enabled">
+                        <span class="toggle-slider"></span>
+                    </label>
+                </div>
+            </div>
+        </div>
+{{template "admin/layout/footer.html" .}}