docs: add architecture and settlement overview

[vku2018]

Summary

• add docs/architecture.md covering the Express composition root, middleware flow, in-memory stores, audit events, and pause/read behavior
• document the record-usage -> quote -> settle lifecycle with Mermaid sequence diagrams
• clarify that POST /api/v1/settle is an off-chain accounting drain and that durable storage/on-chain settlement are planned integration points
• link the new architecture doc from the README documentation section

Closes #56

Validation

• PASS: npm run build
• PASS: npm run lint
• PASS: tracked-source test run after build: $env:NODE_ENV='test'; node --test <tracked dist test files> (89 passing)
• PASS: npx prettier --check docs/architecture.md
• PASS: git diff --check
• NOTE: npm test fails on Windows PowerShell before running tests because the script uses POSIX inline NODE_ENV=test.
• NOTE: npm run format remains a repo-wide baseline failure across existing files; the new architecture doc passes Prettier directly.