Add debug logging for received image data

Log type, length, and first 50 chars of the received base64 string
to diagnose truncation or format issues.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

api_server.py

@@ -472,6 +472,7 @@ def create_fastapi_app():
             return JSONResponse({"error": "Expected 7 parameters in data array"}, status_code=400)
 
         image_base64 = data[0]
+        print(f"  Received image string: type={type(image_base64)}, len={len(image_base64) if isinstance(image_base64, str) else 'N/A'}, first 50 chars: {str(image_base64)[:50]}")
         auto_axis = bool(data[1])
         downsample_mode = str(data[2])
         max_points = int(data[3])