[sam2][demo][1/x] Fix file upload

Summary:

The Strawberry GraphQL library recently disabled multipart requests by default. This resulted in a video upload request returning "Unsupported content type" instead of uploading the video, processing it, and returning the video path.

This change enables multipart request support on the endpoint view.

Test Plan:

Tested locally and upload succeeds

demo/backend/server/app.py

@@ -128,6 +128,10 @@ def get_context(self, request: Request, response: Response) -> Any:
         # https://strawberry.rocks/docs/operations/deployment
         # https://strawberry.rocks/docs/integrations/flask
         allow_queries_via_get=False,
+        # Strawberry recently changed multipart request handling, which now
+        # requires enabling support explicitly for views.
+        # https://github.com/strawberry-graphql/strawberry/issues/3655
+        multipart_uploads_enabled=True,
     ),
 )