Add delegated agent checkout and USDC payments

[solomon2773]

Summary

• add customer-created, expiring agent grants with exact product/currency allowlists and spend caps
• publish a machine-readable catalog and idempotent native checkout API
• add ACP 2026-04-17 checkout sessions with Stripe Shared Payment Token charging
• add six-decimal USDC accounting and exact x402 v2 verification/settlement with replay protection
• preserve metered usage precision and use serializable spend reservation under concurrency
• keep anonymous autonomous account creation in the future roadmap

Validation

• 91 unit tests
• 14 PostgreSQL integration tests
• Prisma validation and strict TypeScript check
• production Next.js build

Stacked on #27.

Closes #11