perf(volcano): preserve source quality for inline retry images

lib/ai/providers/volcano-vision.ts

@@ -53,6 +53,11 @@ async function fetchImageAsDataUri(url: string): Promise<string | null> {
 
     const inputMime = response.headers.get("content-type") || guessImageMimeTypeFromUrl(url);
 
+    // 优先保留原图，避免 base64 回退路径带来明显画质损失
+    if (buffer.length <= 6 * 1024 * 1024) {
+      return `data:${inputMime};base64,${buffer.toString("base64")}`;
+    }
+
     try {
       const sharp = await import("sharp");
       const optimized = await sharp.default(buffer)
@@ -61,7 +66,7 @@ async function fetchImageAsDataUri(url: string): Promise<string | null> {
           fit: "inside",
           withoutEnlargement: true,
         })
-        .jpeg({ quality: 86, mozjpeg: true })
+        .jpeg({ quality: 90, mozjpeg: true })
         .toBuffer();
       return `data:image/jpeg;base64,${optimized.toString("base64")}`;
     } catch {