Introduce Stripe integration for automatic payment ingestion and refund
tracking. Adds new fields to the payment model for Stripe IDs and
status,
Stripe client driver, sync service, cron job, manual API endpoint, and
public webhook handler for real-time updates. Includes tests and
documentation. Manual cash entry remains supported.
This commit standardizes the naming of identifier and API key fields
across the codebase to use consistent camel case (e.g., `ID`, `APIKey`,
`DatabaseURL`). This includes updates to struct fields, method names,
function parameters, and environment variable references. The changes
improve code clarity and maintainability by reducing ambiguity and
aligning with Go naming conventions. No functional behavior is changed.
