fix: 删书彻底清理文件与DB + 阅读位置跨设备同步

删除不彻底的根因是库里 file_path 存了旧 Windows 相对路径（uploads\xxx.epub），Linux 容器里 os.path.exists 永远为假：

- 新增 _upload_file() 归一化路径（取文件名挂到当前 uploads 目录），读/删都走它，老库迁移的文件也能定位；文件删除挪到后台任务，失败不影响接口

- 删除后对 SQLite 执行 VACUUM，让 db 文件真正缩小

- 删除时一并清理 reading_progress

阅读位置原来只存 localStorage，换设备丢失：

- 新增 reading_progress 表 + GET/PUT /books/{id}/progress 接口

- 前端打开时优先取服务端位置（本地兜底并双端拉齐），翻页防抖同步到服务端，关闭时把防抖窗口内的最后一页立即刷上去

frontend/src/components/EpubReader.tsx

@@ -184,7 +184,16 @@ export default function EpubReader({ book, onClose, onAsk }: {
         setToc(entries);
         setFontSize(initialFont);
         const host = viewerRef.current!;
-        const savedCfi = parseSavedCfi(readSaved(positionKey));
+        // 阅读位置优先取服务端（跨设备同步），本地 localStorage 只做离线兜底
+        const localCfi = parseSavedCfi(readSaved(positionKey));
+        let savedCfi = localCfi;
+        try {
+          const progress = await api.getReadingProgress(book.id);
+          if (progress?.cfi) {
+            savedCfi = progress.cfi;
+            if (localCfi !== progress.cfi) save(positionKey, progress.cfi); // 双端拉齐
+          }
+        } catch { /* 拉取失败不阻断打开，用本地位置 */ }
         restoringPositionRef.current = true;
         cfiRef.current = savedCfi;
         setContinuous(false);
@@ -255,7 +264,12 @@ export default function EpubReader({ book, onClose, onAsk }: {
           cfiRef.current = start.cfi;
           if (restoringPositionRef.current) return;
           window.clearTimeout(saveTimer);
-          saveTimer = window.setTimeout(() => { if (!disposed) save(positionKey, start.cfi!); }, 300);
+          saveTimer = window.setTimeout(() => {
+            if (disposed) return;
+            save(positionKey, start.cfi!);
+            // 同步到服务端（跨设备）；失败静默，下次 relocate 会再试
+            void api.saveReadingProgress(book.id, start.cfi!).catch(() => {});
+          }, 300);
         });
         rendition.on("selected", (cfi: string, contents: Contents) => {
           if (disposed) return;
@@ -299,6 +313,11 @@ export default function EpubReader({ book, onClose, onAsk }: {
     return () => {
       disposed = true;
       controller.abort();
+      // 关书页时把防抖窗口里的位置立即刷到服务端，避免「最后一页丢失」
+      if (cfiRef.current && !restoringPositionRef.current) {
+        save(positionKey, cfiRef.current);
+        void api.saveReadingProgress(book.id, cfiRef.current).catch(() => {});
+      }
       window.clearTimeout(saveTimer);
       observer?.disconnect();
       renditionRef.current = null;

frontend/src/lib/api.ts

@@ -27,6 +27,12 @@ export interface ReadingNote {
   updated_at: string;
 }
 
+export interface ReadingProgress {
+  book_id: string;
+  cfi: string;
+  updated_at: string;
+}
+
 export interface Chapter {
   id: string;
   book_id: string;
@@ -161,6 +167,10 @@ export const api = {
     return res.json() as Promise<Book>;
   },
   deleteBook: (id: string) => request<{ deleted: string }>(`/books/${id}`, { method: "DELETE" }),
+  // 阅读位置（跨设备同步）：服务端存最近位置，本地 localStorage 只做离线兜底
+  getReadingProgress: (bookId: string) => request<ReadingProgress | null>(`/books/${bookId}/progress`),
+  saveReadingProgress: (bookId: string, cfi: string) =>
+    request<ReadingProgress>(`/books/${bookId}/progress`, { method: "PUT", body: JSON.stringify({ cfi }) }),
 
   // Conversations
   listConversations: (bookId?: string, kind?: string) => {