feat: 社区准则改为数据库存储

- AuthController 注入 SiteSettings 依赖，RegisterPage 优先从 DB 读取准则
- DB 中 site.guidelines 为空时 fallback 到静态 guidelines.html 文件
- 管理后台站点设置页新增「内容管理」分区，textarea 编辑社区准则
- 添加准则编辑区垂直布局样式

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

@@ -58,6 +58,35 @@
 .btn-save { width: 56px; text-align: center; }
 .btn-save:disabled { opacity: 0.5; cursor: not-allowed; }
 
+/* --- 垂直布局设置项（用于准则等大文本） --- */
+.setting-item-vertical {
+    flex-direction: column; align-items: stretch;
+}
+.setting-item-vertical .setting-info {
+    margin-bottom: 10px;
+}
+.setting-input-block {
+    display: flex; flex-direction: column; gap: 10px;
+}
+.setting-textarea {
+    padding: 10px 12px;
+    border: 1px solid var(--border);
+    border-radius: 4px;
+    font-size: 13px; line-height: 1.6;
+    font-family: 'Courier New', Consolas, monospace;
+    resize: vertical;
+    width: 100%;
+    box-sizing: border-box;
+    outline: none;
+    transition: border-color 0.2s;
+}
+.setting-textarea:focus { border-color: var(--primary); }
+.btn-save-block {
+    align-self: flex-end;
+    min-width: 80px;
+}
+.btn-save-block:disabled { opacity: 0.5; cursor: not-allowed; }
+
 /* --- 空状态 --- */
 .settings-empty {
     text-align: center; padding: 28px 20px;