[codex] Align bulk usage validation

[pq198363-ops]

Closes #89.

Summary

• extract a shared validateUsageItem helper so single and bulk usage ingestion share identifier and request-count validation
• reject disabled services per item in POST /api/v1/usage/bulk with error: "service_disabled"
• preserve the bulk partial-success contract so valid rows still accumulate while invalid or disabled rows are reported by index
• document the bulk validation and disabled-service behavior

Verification

• RED: npm run build; $env:NODE_ENV='test'; node --test dist/usage-bulk-parity.test.js failed before implementation because disabled services and empty/oversized identifiers were accepted by the bulk endpoint
• GREEN focused: npm run build; $env:NODE_ENV='test'; node --test dist/usage-bulk-parity.test.js passed 2 tests
• Full: npx prettier --check README.md src/routes/usage.ts src/usage-bulk-parity.test.ts
• Full: npm run lint
• Full: clean dist/, npm run build, then $env:NODE_ENV='test'; node --test dist/*.test.js dist/**/*.test.js passed 91 tests across 11 suites
• Full: git diff --check