fix: remove ImageNet normalization for YOLO (match Ultralytics default)

- Ultralytics YOLO only does /255 normalization, no ImageNet mean/std
- This was causing detection box coordinate drift

internal/captcha/handler.go

@@ -982,19 +982,17 @@ func preprocessYOLO(img image.Image) ([]float32, LetterboxInfo, error) {
 		}
 	}
 
-	// YOLO 标准化参数 (ImageNet)
-	mean := [3]float32{0.485, 0.456, 0.406}
-	std := [3]float32{0.229, 0.224, 0.225}
-
+	// YOLO 归一化：只需要 /255，不需要 ImageNet 标准化
+	// Ultralytics YOLO 默认预处理：letterbox + /255
 	pixels := make([]float32, 3*targetSize*targetSize)
 	for y := 0; y < targetSize; y++ {
 		for x := 0; x < targetSize; x++ {
 			c := canvas.At(x, y)
 			r, g, b, _ := c.RGBA()
-			// 归一化到 [0, 1]，然后标准化
-			pixels[0*targetSize*targetSize+y*targetSize+x] = (float32(r)/65535.0 - mean[0]) / std[0]
-			pixels[1*targetSize*targetSize+y*targetSize+x] = (float32(g)/65535.0 - mean[1]) / std[1]
-			pixels[2*targetSize*targetSize+y*targetSize+x] = (float32(b)/65535.0 - mean[2]) / std[2]
+			// 归一化到 [0, 1]
+			pixels[0*targetSize*targetSize+y*targetSize+x] = float32(r) / 65535.0
+			pixels[1*targetSize*targetSize+y*targetSize+x] = float32(g) / 65535.0
+			pixels[2*targetSize*targetSize+y*targetSize+x] = float32(b) / 65535.0
 		}
 	}