refactor: break up code into sensible modules

[cabljac]

This PR is refactoring and moving code about to make it more maintainable, readable, and testable.

[Copilot]

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• firestore-stripe-invoices/functions/package.json: Language not supported
• firestore-stripe-payments/functions/package.json: Language not supported

Comments suppressed due to low confidence (1)

firestore-stripe-payments/functions/src/controllers/checkout.ts:259

• [nitpick] The error message for unsupported client types contains an extra space before 'mobile'. Consider revising it to: "Only 'web' or 'mobile' is supported!" for clarity.

throw new Error(`Client ${client} is not supported. Only 'web' or ' mobile' is supported!`);