Create a realtime WebRTC call

OpenAI-compatible WebRTC SDP exchange. Accepts a raw application/sdp offer with the model in the ?model= query parameter, or a multipart form with sdp and session (JSON) fields. The gateway routes by model, injects provider credentials, and relays the SDP answer; the Location header carries the created call id. Media flows directly between the client and the provider, so usage is recorded by a gateway-side sideband observer when usage tracking is enabled.