The web UI is multi-instance and can connect to the backend at a
different origin (the direct :8000 port, a LAN IP, 127.0.0.1 vs
localhost), which the browser blocks without CORS headers. Adds
CORSMiddleware driven by a new cors_allow_origins setting (default "*",
safe here: bearer-token auth with allow_credentials=False). Accepts a
comma-separated string in .env.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Replace the placeholder MUSICBRAINZ_USER_AGENT env var with
MUSICBRAINZ_OWNER_EMAIL. The User-Agent ("MCMA/<version> ( <contact> )")
is now composed from the fixed app name, the installed package version,
and the operator's contact email — falling back to the project URL when
no email is configured. Also use the same version for the FastAPI app.