84013c0e

By: Michael Lynch <git@mtlynch.io>

Decouple upload handler tests from the media service

Replace the real media service and processor test doubles with a
testMediaPublisher that implements handlers.MediaPublisher, so upload
handler tests exercise only the handler's own behavior.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
configuredevShellx86_64-linuxbackendbackend-devcheck-bashcheck-frontendcheck-go-formattingcheck-go-test-packagescheck-trailing-newlinecheck-trailing-whitespacedeploy-to-flydiagramdocker-imagee2e-testsgo-testsimport-from-tinybeansjs-testslint-sqllittle-momentslittle-moments-devx86_64-linux