docs(billing): add module README

[PierreBrisorgueil]

Summary

• Add billing module README with quota system documentation
• Add commented-out quota config example in development config
• Covers: configuration, requireQuota middleware, BillingUsageService, usage endpoint, plan change events

Test plan

• npm run lint passes
• npm test passes (264 tests)

[Copilot]

Pull request overview

Adds module-level documentation for the billing/quota system and a small inline config example to make downstream configuration clearer.

Changes:

• Added modules/billing/README.md documenting quota configuration, middleware usage, usage tracking, usage endpoint, and plan change events.
• Added a commented-out quota configuration example in modules/billing/config/billing.development.config.js.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
modules/billing/config/billing.development.config.js	Adds an inline commented example showing how downstream projects can define per-plan quotas.
modules/billing/README.md	Introduces billing module documentation and usage examples for quotas, usage tracking, and events.

[coderabbitai]

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: ASSERTIVE

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Run ID: 7ca20e31-567d-4d47-b915-743ad8895ac3

📥 Commits

Reviewing files that changed from the base of the PR and between 4b0938c and cafea9c.

📒 Files selected for processing (1)

• modules/billing/README.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch docs/billing-readme

📝 Coding Plan

• Generate coding plan for human review comments

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