Answer a ringing or early-media inbound SIP leg (asynchronous)

Signals the inbound-call goroutine to send 200 OK. The HTTP call returns 202 immediately; the actual SIP 200 OK is sent in the background, and the leg's transition is observed via leg.connected. Pre-condition failures (wrong state, unknown codec) still return 4xx synchronously. When the caller offers several m=audio sections they are all accepted; streams optionally routes each accepted stream beyond the primary into its own room, applied once the answer is negotiated.