Connect a MoQ (Media over QUIC) leg (WebTransport extended-CONNECT, experimental)

Actual HTTP method: CONNECT (HTTP/3 extended-CONNECT for WebTransport). OpenAPI 3.1 does not define connect as a path-item method, so this operation is documented under post with an x-actual-method: CONNECT extension. Standard HTTP clients (e.g. curl -X POST) will receive 405 Method Not Allowed — use a WebTransport-capable HTTP/3 client.

Experimental / PoC. Upgrades an HTTP/3 extended-CONNECT request to a WebTransport session and creates an inbound MoQ leg. Reachable only over HTTP/3 on the MoQ listener (not on the regular HTTP/1.1 chi listener). Requires MOQ_ENABLED=true plus MOQ_TLS_CERT_FILE and MOQ_TLS_KEY_FILE. Speaks IETF draft-11 of moq-transport (via mengelbart/moqtransport); browser interop with draft-16 clients (moqtail, moq.dev) is not expected to work. Media is bidirectional within a single MoQ session per leg: the server publishes namespace mix/track audio (downlink, room mix) and subscribes to namespace mic/track audio (uplink, leg mic). Audio is Opus framed one frame per MoQ Object (LOC-style) at 48 kHz mono, 20 ms frames. Query parameters: sample_rate (only 48000 is accepted — the encoder/decoder are hard-wired to 48 kHz; default 48000); room_id to auto-add the leg to a room; app_id for event filtering; webhook_url/webhook_secret for per-leg event routing. X-* and P-* request headers (plus Authorization) are captured into the leg's headers map and surfaced on LegView. The leg goes straight to connected (no ringing/answer flow); no DTMF, no RTT, and event parity is limited to leg.connected / leg.disconnected.