[codex] Reject unknown services in billing

[pq198363-ops]

Fixes #40.

Summary

• returns structured 404 not_found from GET /api/v1/billing/:agent/:serviceId when the service is not registered
• returns structured 404 not_found from POST /api/v1/settle for unknown services before touching the usage accumulator
• preserves registered zero-price service behavior and existing success response shapes
• documents the behavior in README and the changelog endpoint

Verification

• RED: npm run build; $env:NODE_ENV='test'; node --test dist/unknown-service-billing.test.js failed before implementation because unknown service billing and settlement returned 200.
• GREEN focused: npm run build; $env:NODE_ENV='test'; node --test dist/unknown-service-billing.test.js passed 3 tests.
• npx prettier --check README.md src/routes/usage.ts src/routes/meta.ts src/unknown-service-billing.test.ts
• npm run lint
• clean dist/, then npm run build
• $env:NODE_ENV='test'; node --test dist/*.test.js dist/**/*.test.js passed 92 tests across 11 suites.
• git diff --check

Data integrity note

The settlement path now verifies the service exists before reading and resetting the usage key, so an unknown service cannot drain metered usage at a zero price. Registered services with priceStroops: 0 still quote and settle successfully.