feat: use bigint for stroops billing

[pq198363-ops]

Closes #41

Summary

• adds a BigInt-backed stroops helper for exact billing multiplication and summation
• returns billedStroops, totalStroops, and disabledStroops as decimal strings so JSON responses do not lose precision above 2^53
• applies the same exact arithmetic to pair billing, settlement, and protocol-wide billing totals
• documents the breaking response-type change in README and billing docs
• adds OpenAPI metadata for the string-typed billing response fields

TDD evidence

• RED helper contract: npm run build failed because src/util/stroops.ts did not exist.
• RED API contract: $env:NODE_ENV='test'; node --test dist/billing-stroops.test.js failed because billing endpoints returned numbers/scientific notation instead of exact decimal strings.
• GREEN: focused helper and API billing tests passed after routing billing math through BigInt helpers.

Validation

• npx prettier --check README.md docs/billing-units.md src/routes/usage.ts src/routes/meta.ts src/routes/operational.test.ts src/billing-stroops.test.ts src/billing-total-breakdown.test.ts src/routes/usage.test.ts src/health.test.ts src/util/stroops.ts src/util/stroops.test.ts
• npm run lint
• clean dist/, then npm run build
• $env:NODE_ENV='test'; node --test dist/*.test.js dist/**/*.test.js
• git diff --check

Full clean test summary: 96 tests, 12 suites, 96 pass, 0 fail.

Note: the package npm test script uses POSIX-style NODE_ENV=test, so on Windows PowerShell I ran the equivalent build plus node --test command above.