Connect a WebSocket as a leg (HTTP upgrade)

Upgrades the HTTP request to a WebSocket and creates a websocket_in leg. Query parameters: sample_rate (8000/16000/24000/48000; default 16000); wire_format (binary default, or json_base64); sample_format (s16le only in v1); room_id to auto-add the leg to a room; app_id for event filtering; rtt=true to enable the bidirectional text channel; 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 leg.ringing and in LegView. The leg goes straight to connected (no ringing/answer flow). Audio frames carry PCM16-LE mono at sample_rate; with wire_format=binary each WebSocket binary frame is exactly one 20ms PCM frame, with json_base64 the same payload is wrapped as {"type":"audio","audio":"<base64>"}. Text and control messages always use JSON text frames: {"type":"text","text":...}, {"type":"ping","event_id":N}/{"type":"pong","event_id":N}, and {"type":"hangup"} to terminate from the peer side.