feat: 新增备用接口机制提升截图服务高可用性

当 mShots 返回无效图片或失败时，自动降级尝试使用 thum.io 获取截图。主要修改包括：
- 新增 fallback 请求处理逻辑
- 扩展缓存元数据记录来源信息
- 在响应头中添加 X-Source 标识备用接口

server.js

@@ -9,6 +9,7 @@ require('dotenv').config();
 const app = express();
 const PORT = Number(process.env.PORT) || 11489;
 const UPSTREAM_HOST = 'https://s0.wp.com';
+const FALLBACK_HOST_BASE = 'https://image.thum.io/get/width/1024/crop/768/noanimate';
 const CACHE_DIR = path.join(process.cwd(), 'cache');
 
 fs.mkdirSync(CACHE_DIR, { recursive: true });
@@ -145,11 +146,37 @@ async function fetchUpstreamWithRetry(upstreamUrl, tries = 2) {
 }
 
 /**
- * 核心处理逻辑：检查缓存 -> 回源 -> 写入缓存 -> 返回响应
+ * 请求备用接口 (thum.io)
+ * @param {string} targetUrl 
+ * @returns {Promise<any>}
+ */
+async function fetchFallbackWithRetry(targetUrl) {
+  // thum.io 格式: https://image.thum.io/get/<options>/<url>
+  const fallbackUrl = `${FALLBACK_HOST_BASE}/${targetUrl}`;
+  console.log(`[fallback-request] trying fallback: ${fallbackUrl}`);
+  
+  try {
+    return await axios.get(fallbackUrl, {
+      responseType: 'arraybuffer',
+      headers: {
+        'User-Agent': 'Mozilla/5.0 (Windows NT 10.0; Win64; x64)',
+      },
+      timeout: 30000,
+      validateStatus: () => true,
+    });
+  } catch (err) {
+    console.error(`[fallback-error] msg=${err.message}`);
+    return null;
+  }
+}
+
+/**
+ * 核心处理逻辑：检查缓存 -> 回源 -> (失败则) 备用接口 -> 写入缓存 -> 返回响应
  * @param {object} res 
  * @param {string} upstreamUrl 
+ * @param {string} targetUrl
  */
-async function handleProxyRequest(res, upstreamUrl) {
+async function handleProxyRequest(res, upstreamUrl, targetUrl) {
   const key = sha1(upstreamUrl);
   const { data: dataPath, meta: metaPath } = getCachePaths(key);
 
@@ -176,27 +203,48 @@ async function handleProxyRequest(res, upstreamUrl) {
     // 2. 回源请求
     const resp = await fetchUpstreamWithRetry(upstreamUrl);
 
-    // 3. 仅缓存有效图片
-    if (isValidImageResponse(resp.status, resp.headers, resp.data)) {
-      const contentType = resp.headers['content-type'] || 'image/jpeg';
+    // 3. 检查响应是否有效
+    let finalResp = resp;
+    let isFallback = false;
+
+    if (!isValidImageResponse(resp.status, resp.headers, resp.data)) {
+       console.log(`[upstream-invalid] url=${upstreamUrl} status=${resp.status} len=${resp.data.byteLength}, trying fallback...`);
+       const fallbackResp = await fetchFallbackWithRetry(targetUrl);
+       
+       if (fallbackResp && isValidImageResponse(fallbackResp.status, fallbackResp.headers, fallbackResp.data)) {
+         console.log(`[fallback-success] url=${targetUrl}`);
+         finalResp = fallbackResp;
+         isFallback = true;
+       } else {
+         console.log(`[fallback-failed] url=${targetUrl}, returning original response`);
+       }
+    }
+
+    // 4. 仅缓存有效图片 (无论是回源还是备用)
+    if (isValidImageResponse(finalResp.status, finalResp.headers, finalResp.data)) {
+      const contentType = finalResp.headers['content-type'] || 'image/jpeg';
       const meta = {
-        url: upstreamUrl,
+        url: isFallback ? `fallback:${targetUrl}` : upstreamUrl,
         contentType,
-        size: resp.data.byteLength,
+        size: finalResp.data.byteLength,
         createdAt: new Date().toISOString(),
+        source: isFallback ? 'thum.io' : 'mshots'
       };
       try {
-        fs.writeFileSync(dataPath, resp.data);
+        fs.writeFileSync(dataPath, finalResp.data);
         fs.writeFileSync(metaPath, JSON.stringify(meta));
       } catch (e) {
         // 写盘失败也继续返回
       }
       res.type(contentType);
       res.set('Cache-Control', 'public, max-age=315360000, immutable');
-      return res.send(resp.data);
+      if (isFallback) {
+         res.set('X-Source', 'fallback-thum.io');
+      }
+      return res.send(finalResp.data);
     }
 
-    // 4. 非图片或非200：兜底策略
+    // 5. 非图片或非200：兜底策略
     // 若本地已有有效缓存（可能是旧的但有效），优先返回缓存
     if (fs.existsSync(dataPath) && fs.existsSync(metaPath)) {
       try {
@@ -263,7 +311,7 @@ app.use('/mshots/v1', async (req, res) => {
   // 拼接完整上游 URL
   const upstreamUrl = UPSTREAM_HOST + '/mshots/v1/' + targetUrl;
   
-  return handleProxyRequest(res, upstreamUrl);
+  return handleProxyRequest(res, upstreamUrl, targetUrl);
 });
 
 // 根路径处理：/https://example.com 或 /www.baidu.com
@@ -281,7 +329,7 @@ app.use(async (req, res) => {
   // 拼接完整上游 URL
   const upstreamUrl = UPSTREAM_HOST + '/mshots/v1/' + targetUrl;
 
-  return handleProxyRequest(res, upstreamUrl);
+  return handleProxyRequest(res, upstreamUrl, targetUrl);
 });
 
 // 简单根路由健康检查