Open a relayed Gladia live-transcription session

Creates a Gladia live session server-side and returns a WebSocket URL on this API. The client never sees the Gladia key and never contacts api.gladia.io — which is what makes dictation work from networks that block it. Connect to relay_url within expires_in seconds. The ticket is scoped to this one session and to the calling user, and is NOT single-use: verification is stateless (HMAC only) so that any api replica can serve the socket, which means the same ticket opens more than one relay socket while it is still valid. Concurrency is bounded instead — the relay refuses a connection with close code 4429 once a user already holds the per-user maximum of live relay sockets.