The Synapse backend and memory service bound 0.0.0.0 with wildcard CORS and no
auth, exposing the full unauthenticated admin/data API to the LAN. Default the
uvicorn bind to 127.0.0.1 (NEXUS_BIND_HOST override), scope CORS to known local
origins instead of "*", and add TrustedHostMiddleware to reject foreign Host
headers (which defeats DNS-rebinding, something same-origin CORS cannot stop).
NEXUS_ALLOWED_HOSTS / NEXUS_ALLOWED_ORIGINS allow opt-in LAN exposure, intended
to be paired with real authentication.
Co-authored-by: Cursor <cursoragent@cursor.com>