docs: add demo-mode payment evidence documentation and fix build errors

[Emoji-dot]

• Add comprehensive documentation explaining demo vs real payment evidence
• Document demo request flow with 402 challenge and retry semantics
• Include minimal curl example using x-query402-demo-paid header
• Link documentation from README
• Fix TypeScript build errors:
  • Add PaymentPayload type import and assertion in x402.ts
  • Replace Node.js Buffer with browser-compatible base64 encoding in web/sponsorship.ts
  • Exclude test files from web app typecheck to avoid node:test import errors
  • Fix Windows path separator issue in memory.test.ts

All CI checks pass: typecheck, build, test, lint, format, source-artifacts
close #27

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[Emoji-dot]

Please review

[emrekayat]

Thanks for the detailed demo evidence doc. I reviewed this against #27 and ran:\n\n- npm run format:check -- README.MD docs/demo-payment-evidence.md apps/api/src/lib/storage/memory.test.ts apps/web/tsconfig.json\n- npm test --workspace @query402/api -- src/lib/storage/memory.test.ts\n- npm run typecheck --workspace @query402/api\n- npm run typecheck --workspace @query402/web\n\nThose checks pass, but I cannot merge yet because a few things need cleanup:\n\n1. The new docs use http://localhost:3000 for API examples. The API default in this repo is http://localhost:3001 (PORT_API and API_BASE_URL defaults), so the curl examples should use port 3001.\n2. The /api/usage response example is not shaped like the current route. The route returns { usage, pagination: { limit, offset, count } }, not top-level total, limit, and offset.\n3. package-lock.json has a very large dependency churn, but no package manifest changed. Please remove the lockfile changes unless there is a specific package update required for this issue.\n\nOnce those are corrected, the documentation piece should be much easier to merge.

[Emoji-dot]

Okay sir