fix(dify): send 'user' as plain form field in file upload

The multipart tuple form (None, user) is httpx 'files=' syntax for a part
with no filename; placed under 'data=' it expanded into a stray user=None
field, so Dify associated the uploaded file with the wrong user and the
workflow never received the image. Send 'user' as a plain string.

src/langbot/libs/dify_service_api/v1/client.py

@@ -145,7 +145,7 @@ class AsyncDifyServiceClient:
                     'file': file,
                 },
                 data={
-                    'user': (None, user),
+                    'user': user,
                 },
             )