fix: re-dump sponsors fixture with --natural-foreign

Re-dumped using `dumpdata --natural-foreign` so content_type,
polymorphic_ctype, and user FK references use natural keys
instead of integer IDs. Addresses ewdurbin's review comment.

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

Author: JacobCoffee