WaveSurfer makes plain browser fetches without Authorization headers,
causing 401s on the stream endpoint. The stream endpoint now accepts
a ?token= query param in addition to the Authorization header, and
proxies audio bytes directly through FastAPI instead of redirecting to
raw WebDAV (which would require a second Nextcloud auth challenge).
Falls back to nc_file_path if HLS transcoding hasn't run yet.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>