docs: add customer_billing app reference and update lifecycle doc

[iloveagent57]

Add customer-billing-app-reference.rst covering the app module structure, models, Stripe event handlers, Celery tasks, management commands, and external service dependencies.

Update subscription-and-renewal-lifecycle.rst with gaps identified during the architecture review:

• Expand CheckoutIntent state field to show full state machine including ERRORED_FULFILLMENT_STALLED and recovery paths
• Document checkout_intent_uuid vs checkout_intent_id lookup fallback on StripeEventData, and add handled_at field description
• Explain the invoice.created / invoice.paid two-event split and out-of-order delivery handling in the trial-to-paid section
• Add Management Commands section

Motivation: qualitative exploration of whether https://github.com/abhigyanpatwari/GitNexus can help LLMs more thoroughly and correctly read, trace, and explain our code.

Merge checklist:

• ./manage.py makemigrations has been run
  • Note: This must be run if you modified any models.
    • It may or may not make a migration depending on exactly what you modified, but it should still be run.

Post merge:

• Ensure that your changes went out to the stage instance
• Deploy to prod instance

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.06%. Comparing base (bc44b59) to head (9b60c55).

Additional details and impacted files

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  Coverage   87.06%   87.06%           
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  Files         155      155           
  Lines       13014    13014           
  Branches     1249     1249           
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  Hits        11331    11331           
  Misses       1375     1375           
  Partials      308      308           

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[Copilot]

Pull request overview

This PR expands the Customer Billing documentation set by adding a module-level reference for the customer_billing Django app and tightening the subscription/renewal lifecycle narrative to reflect additional states, webhook processing details, and operational tooling.

Changes:

• Added customer-billing-app-reference.rst describing core models, key modules, Celery tasks, management commands, and external dependencies.
• Expanded subscription-and-renewal-lifecycle.rst with a fuller CheckoutIntent.state state machine, StripeEventData.handled_at + checkout intent lookup behavior, and detailed handling for invoice.created/invoice.paid ordering.
• Documented operational management commands in the lifecycle doc.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
docs/customer-billing/subscription-and-renewal-lifecycle.rst	Expands lifecycle/state-machine and webhook-processing docs; adds management commands section.
docs/customer-billing/customer-billing-app-reference.rst	New module-level reference for the customer_billing app structure, behavior, and dependencies.