Estimate call cost

Pre-call cost estimate for a PSTN call: the carrier leg plus optional recording and transcription add-ons. Same billing formula as the post-call invoice, so the quote and the final charge can't disagree. The per-minute figure is deliberately conservative (the real cost comes from the settled carrier record after the call), so estimates trend slightly over the actual invoice. Parity endpoint of GET /v1/whatsapp/calls/estimate, minus the Meta line (PSTN calls have no separate Meta bill, so totalCostUSD equals billableCostUSD).