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/library/LibraryCoverPicker.tsx

@@ -4,8 +4,35 @@ import { Images, Loader2 } from "lucide-react";
 import { useEffect, useState } from "react";
 import Modal from "@/components/Modal";
 import { apiMyMedia, type MediaAttachment } from "@/lib/api";
+import { isOwnImage, variantURL, widthSrcSet } from "@/lib/responsiveImage";
 
-/** 书籍封面图库选择弹窗：列出本人媒体库图片，选中后由父组件复制一份作封面素材 */
+/** 来源标签映射 */
+const SOURCE_LABELS: Record<string, string> = {
+  library_cover: "书籍封面",
+  library_content: "书籍内容",
+  post: "帖子插图",
+};
+
+function sourceLabel(source?: string): string {
+  return SOURCE_LABELS[source || "post"] || source || "帖子插图";
+}
+
+/** 按来源分组（library_cover 优先，其次 library_content，最后 post/其他） */
+function groupBySource(items: MediaAttachment[]): [string, MediaAttachment[]][] {
+  const groups = new Map<string, MediaAttachment[]>();
+  for (const item of items) {
+    const key = item.source || "post";
+    if (!groups.has(key)) groups.set(key, []);
+    groups.get(key)!.push(item);
+  }
+  // 按优先级排序
+  const order = ["library_cover", "library_content", "post"];
+  return Array.from(groups.entries()).sort(
+    ([a], [b]) => order.indexOf(a) - order.indexOf(b)
+  );
+}
+
+/** 书籍封面图库选择弹窗：列出本人媒体库图片，按来源分组 */
 export default function LibraryCoverPicker({
   open,
   onClose,
@@ -45,13 +72,15 @@ export default function LibraryCoverPicker({
     };
   }, [open]);
 
+  const groups = groupBySource(items);
+
   return (
     <Modal
       open={open}
       title="从图库选择封面"
-      description="选择后会复制一份作为封面素材，之后删除原图不影响封面。"
+      description="已选作封面的图片会单独分组，再次选择直接复用。"
       onClose={onClose}
-      maxWidthClass="max-w-lg"
+      maxWidthClass="max-w-2xl"
       dismissOnOverlay
     >
       {loading ? (
@@ -67,28 +96,52 @@ export default function LibraryCoverPicker({
           图库还没有图片。上传封面或发帖插图后，会自动出现在这里。
         </p>
       ) : (
-        <div className="grid grid-cols-4 sm:grid-cols-5 gap-2 max-h-[46vh] overflow-y-auto">
-          {items.map((item) => (
-            <button
-              key={item.id}
-              type="button"
-              onClick={() => {
-                onPick(item);
-                onClose();
-              }}
-              className="relative group aspect-square rounded-xl overflow-hidden"
-              style={{ boxShadow: "0 0 0 1px var(--line)", background: "var(--panel-2)" }}
-              data-tip="复制一份用作封面"
-            >
-              {/* eslint-disable-next-line @next/next/no-img-element */}
-              <img src={item.url} alt="图库候选封面" 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" }}
+        <div className="flex flex-col gap-5 max-h-[60vh] overflow-y-auto">
+          {groups.map(([source, groupItems]) => (
+            <section key={source}>
+              <h4
+                className="text-[11.5px] font-bold tracking-wide uppercase mb-2 px-0.5"
+                style={{ color: "var(--ink-3)" }}
               >
-                <Images size={14} />
-              </span>
-            </button>
+                {sourceLabel(source)}
+                <span className="ml-1.5 font-normal opacity-70">({groupItems.length})</span>
+              </h4>
+              <div className="grid grid-cols-4 sm:grid-cols-5 gap-2">
+                {groupItems.map((item) => (
+                  <button
+                    key={item.id}
+                    type="button"
+                    onClick={() => {
+                      onPick(item);
+                      onClose();
+                    }}
+                    className="relative group aspect-square rounded-xl overflow-hidden"
+                    style={{ boxShadow: "0 0 0 1px var(--line)", background: "var(--panel-2)" }}
+                    data-tip={
+                      item.source === "library_cover"
+                        ? "已作封面，点击直接复用"
+                        : "复制一份用作封面"
+                    }
+                  >
+                    {/* eslint-disable-next-line @next/next/no-img-element */}
+                    <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="图库候选封面"
+                      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" }}
+                    >
+                      <Images size={14} />
+                    </span>
+                  </button>
+                ))}
+              </div>
+            </section>
           ))}
         </div>
       )}