- Netease-sync/server.js：在 processSong 中新增换源回退逻辑

- 日志会提示：
      - Trying fallback source kuwo for ...
      - No search hit from ...（若搜索无结果）

Netease-sync/server.js

@@ -551,6 +551,34 @@ async function processSong(song) {
             if (fallbackOk) return true;
         }
 
+        // 换源回退：kuwo -> qq
+        const fallbackSources = ['kuwo', 'qq'].filter(s => s !== source);
+        for (const fallbackSource of fallbackSources) {
+            console.log(`[Sync] Trying fallback source ${fallbackSource} for ${song.name}`);
+            const fallbackHit = await searchSongOnTuneHub(song.name, song.artist, fallbackSource);
+            if (!fallbackHit?.id) {
+                console.warn(`[Sync] No search hit from ${fallbackSource} for ${song.name}`);
+                continue;
+            }
+
+            const fallbackHeaders = buildDownloadHeaders(fallbackSource);
+            const fallbackCandidates = buildDownloadCandidates(
+                fallbackSource,
+                fallbackHit.id,
+                fallbackHit.url,
+                fallbackHit.types
+            );
+
+            const fallbackOk = await tryDownloadWithCandidates(
+                song,
+                fallbackSource,
+                baseName,
+                fallbackCandidates,
+                fallbackHeaders
+            );
+            if (fallbackOk) return true;
+        }
+
         return false; // Failed
 }