fix: send RGB image to diagnose(), not grayscale

The upstream diagnose.py expects RGB input for YOLO detection.
Revert to RGB since the detector needs 3 channels.

src/main.py

@@ -47,9 +47,9 @@ async def infer(image: UploadFile = File(...)) -> dict:
         }
     }
     """
-    # 读取图片内容（转灰度，与 diagnose.py 对文件路径的处理逻辑一致）
+    # 读取图片内容
     contents = await image.read()
-    img = Image.open(io.BytesIO(contents)).convert("L")
+    img = Image.open(io.BytesIO(contents)).convert("RGB")
     img_array = np.array(img)
 
     diagnoser = get_diagnoser()