[codex] Centralize usage-store scan helpers

[pq198363-ops]

Summary

Closes #151.

• Add a shared usageScan helper module with parseUsageKey, scanUsageStore, scanByAgent, and scanByService.
• Route usage exports, aggregate billing, agent rollups, service rollups, and stats through the shared parser/scanners.
• Preserve existing response shapes and values while removing duplicated usageStore.entries() parsing loops.
• Add focused tests for helper parsing/scanning plus endpoint parity across exports, agents, services, billing, and stats.

RED/GREEN

RED before implementation:

npm run build

src/usage-scan-helpers.test.ts(...): error TS2307:
Cannot find module './usageScan.js' or its corresponding type declarations.

GREEN after implementation:

npm run build
NODE_ENV=test node --test dist/usage-scan-helpers.test.js

tests 2
pass 2
fail 0

Verification

npx prettier --check src/usageScan.ts src/usage-scan-helpers.test.ts src/routes/usage.ts src/routes/services.ts src/routes/metrics.ts
All matched files use Prettier code style!

npm run lint
eslint .

npm run build
tsc

NODE_ENV=test node --test dist/*.test.js dist/**/*.test.js
tests 91
pass 91
fail 0

git diff --check

Note: on Windows PowerShell I used the direct $env:NODE_ENV='test'; node --test ... equivalent after npm run build because the package npm test script uses POSIX-style NODE_ENV=test syntax.