feat: add Satoshi facilitator (Base)

[Bortlesboat]

Add Facilitator

Summary

• Faciliator Name: Satoshi API
• URL: <https://facilitator.bitcoinsapi.com>
• Website: <https://bitcoinsapi.com>
• Twitter: N/A
• Short description: x402 facilitator for Satoshi API, a Bitcoin fee intelligence API for apps and AI agents. This adds the verified Base facilitator signer and discovery metadata.

Required changes

• Added config file at packages/facilitators/src/facilitators
• Exported in packages/facilitators/src/facilitators/index.ts
• Appended to packages/facilitators/src/lists/all.ts
• Chose a new unique color not used by other facilitators
• Updated README.md with a one-line entry under Facilitators
• dateOfFirstTransaction is correct and matches on-chain history
• Facilitator has at least 10 USDC transfers
• Added image to apps/scan/public
• Make sure we support that chain
• Make sure all EVM addresses are lowercase

Notes

• This PR intentionally includes the Base facilitator signer only.
• I verified Base USDC settlement history starting on 2026-04-06 for 0xe166267c3648b5ca4419f2c58faed8cd4df87d54.
• I did not include the Solana signer yet because the currently visible on-chain history there is only a funding transfer, not qualifying USDC facilitator activity.
• This branch supersedes duplicate follow-up PRs so review can stay in one place.

Verification

• pnpm --filter facilitators types:check
• pnpm --filter facilitators lint
• pnpm --filter facilitators build
• pnpm exec prettier --check packages/external/facilitators/README.md packages/external/facilitators/src/discovery/index.ts packages/external/facilitators/src/facilitators/index.ts packages/external/facilitators/src/facilitators/satoshi.ts packages/external/facilitators/src/lists/all.ts

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[Bortlesboat]

Refreshed this branch with the latest Base-only facilitator config and discovery export. Consolidating follow-up work here so review stays on one thread; duplicate PRs #787 and #793 are being closed as superseded.