Authenticated WHIP signaling proxy for the browser encoder (golive-beta).

The browser never talks to the internal whip MediaMTX instance directly -- it POSTs its SDP offer here instead, exactly like every other authenticated API route (session cookie via get_current_user). The stream key used for the internal WHIP publish is DERIVED from the caller's own session (user.stream_key), never accepted from the request -- so a browser can only ever publish under its own logged-in user's key, structurally, not by a validation check that could have a bug. whip itself has no public port; only this route can reach it.