fix: harden SUMIT checkout routes for production

[BenKalsky]

Summary

• Harden charge route handling for upstream non-2xx and unmapped SUMIT responses
• Add nested request validation before calling SUMIT
• Make webhook query-string secret opt-in and add real multipart parsing
• Add token-safe checkout hook helpers and update README usage
• Bump package version to 0.1.1

Test Plan

• pnpm test
• pnpm typecheck
• pnpm build
• npm pack --dry-run

Notes

• Query-param webhook secrets are now opt-in via verifySumitSharedSecret(secret, { queryParam: "secret" }); header verification remains the default.