feat: 书库导入导出/图片变体/书籍搜索/小组件运行时等

新增：
- 书库导入导出（library_import/library_export）及测试
- 图片变体生成（image_variants）与响应式图片（responsiveImage）
- 书籍搜索（bookSearch）+ BookSearch/LibrarySearchGrid 组件
- 小组件运行时（widgetRuntime）与静态检查（widgetLint）
- 上传缓存中间件（upload_cache）与字体 CSS 提取脚本

其它：
- 后端 handlers/services 全量调整
- 前端页面、组件、库函数与配置更新

frontend/app/admin/settings/BrandMediaPicker.tsx

@@ -5,6 +5,7 @@ import { useEffect, useState } from "react";
 import Modal from "@/components/Modal";
 import { apiMyMedia, type MediaAttachment } from "@/lib/api";
 import { mediaURLFitsSlot, type BrandSlot } from "@/lib/brand";
+import { isOwnImage, variantURL, widthSrcSet } from "@/lib/responsiveImage";
 
 const SLOT_LABEL: Record<BrandSlot, string> = {
   logo_light: "浅色 Logo",
@@ -88,7 +89,14 @@ export default function BrandMediaPicker({
               data-tip="用作草稿预览，点保存后才生效"
             >
               {/* eslint-disable-next-line @next/next/no-img-element */}
-              <img src={item.url} alt={`${SLOT_LABEL[slot]}候选图`} className="w-full h-full object-contain" loading="lazy" />
+              <img
+                src={isOwnImage(item.url) ? variantURL(item.url, 160) : item.url}
+                srcSet={isOwnImage(item.url) ? widthSrcSet(item.url, [96, 160, 256, 384]) : undefined}
+                sizes="(max-width:520px) 23vw, 96px"
+                alt={`${SLOT_LABEL[slot]}候选图`}
+                className="w-full h-full object-contain"
+                loading="lazy"
+              />
               <span
                 className="pointer-events-none absolute inset-0 flex items-center justify-center opacity-0 group-hover:opacity-100"
                 style={{ background: "color-mix(in srgb, var(--ink) 40%, transparent)", color: "#fff" }}

frontend/app/admin/settings/BrandSeoPanel.tsx

@@ -21,6 +21,7 @@ import {
   type BrandSlot,
 } from "@/lib/brand";
 import { toast } from "@/lib/toast";
+import { densitySrcSet, isOwnImage, pickVariantWidth, variantURL } from "@/lib/responsiveImage";
 import BrandMediaPicker from "./BrandMediaPicker";
 
 type SlotDraft = {
@@ -233,8 +234,13 @@ export default function BrandSeoPanel({
       <div key={slot} className={`admin-brand-asset${compact ? " is-compact" : ""}`}>
         <div className={`admin-brand-swatch ${swatchClass}`}>
           {preview ? (
+            // 预览槽（≤56px 高，宽标可能更宽）：本站图走 160/384 密度变体
             // eslint-disable-next-line @next/next/no-img-element
-            <img src={preview} alt="" />
+            <img
+              src={isOwnImage(preview) ? variantURL(preview, pickVariantWidth(160)) : preview}
+              srcSet={isOwnImage(preview) ? densitySrcSet(preview, 160) : undefined}
+              alt=""
+            />
           ) : (
             <span>未设置</span>
           )}