Reserve numbers to close the search-vs-order race

Reserve a list of phone numbers via Telnyx so the user has a window to confirm purchase without losing them to other Telnyx customers.

Telnyx's available_phone_numbers index is non-authoritative — numbers can be listed there yet rejected at order time. This endpoint calls POST /v2/number_reservations to hold the numbers for ~60s (extendable once), making the subsequent purchase deterministic.

Body: {"phone_numbers": ["+351...", "+351..."]}

Returns: { "reservation_id": str, "status": "pending"|"success"|"failure", "expires_at": ISO timestamp, "phone_numbers": [ {"phone_number": "+351...", "status": "success"|"failure"}, ... ] }

Numbers with status="failure" should be excluded from the upcoming order and the user prompted to pick replacements.