backend: S3 storage adapter + storage_uri rename; auth on /stream
webui: real login + listening wired to the backend contract
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Points the workspace at mcma-webui 61dbb1a (feat(upload): wire A8
local track upload to backend).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>