Feature: make facilitator URL configurable

The facilitator is currently hardcoded to `x402.org` in the Lambda@Edge verification and settlement flow. With the x402 ecosystem now listing 27+ facilitators on [x402.org/ecosystem](https://www.x402.org/ecosystem?filter=facilitators), it would be useful to make this configurable — e.g., via an env variable in `seller-infrastructure/.env` or an SSM parameter.

This would let users:
- Test against different facilitators without code changes
- Use mainnet-capable facilitators (x402.org is testnet-only)
- Choose facilitators optimized for their use case (latency, gas sponsorship, network support, etc.)

Happy to contribute a PR if there's interest.

Start by searching the Lambda@Edge verification and settlement flow for the hardcoded x402.org URL, then inspect seller-infrastructure/.env and the available SSM configuration. Determine how the facilitator URL should be supplied without code changes, and verify that both verification and settlement use the selected facilitator.

Written by the indexing model from the issue text.