Fresh SEM first-party USDC checkout source

Canonical checkout: https://agents.backlinks.freshsem.com/
Machine instructions: https://agents.backlinks.freshsem.com/agents.txt
OpenAPI: https://agents.backlinks.freshsem.com/openapi.json
License: MIT (see LICENSE.txt)

Published source files
- checkout.html.txt — browser interface, wallet connection, email recovery, and payment-status polling
- agent_backlinks.php.txt — quote, secure-session, payment-submission, status, and fulfillment endpoint
- agent_backlinks_lib.php.txt — address allocation and strict Base, Polygon, and Solana verification
- agent_backlinks_reconcile.php.txt — server-side pending-payment reconciliation worker
- agent_backlinks_cron_hour.php.txt — recovery integration for the existing hourly Fresh SEM cron
- cron-hour.patch.txt — minimal patch applied to the existing production cron entry point

Security boundary
No private keys, seed phrases, SMTP passwords, database credentials, RPC credentials, or production secret values are included. Runtime secrets live outside the public web source. A wallet-reported hash or signature is not accepted as payment until the server independently verifies the official USDC asset, network, sender, recipient, exact amount, quote time, finality, and proof uniqueness.