Mint a bounded client token

Issues a short-lived, downscoped token (rdc_…) that is safe to ship into an untrusted client app. The token is restricted to the from_numbers (caller IDs) and to_numbers (destinations) you specify, so it can only place calls within those bounds — never from any number to any number. Present it on the same Authorization: Bearer header in place of your API key; the WebRTC SDK accepts it directly. The token's bounds must fall within this key's own number allowlist and its scopes; it is stateless and revoked by expiry. This endpoint requires a real API key — a minted token cannot mint further tokens. Regardless of the parent key's scopes, a client token may only carry the browser-calling scope (voice:webrtc) — it can never make durable account changes or mint further tokens. An explicit, non-empty to_numbers (destination allowlist) is required so the token can never dial an arbitrary number.