Upgrade Stripe SDK from v79 to v83 across the codebase. Update all
imports to use github.com/stripe/stripe-go/v83 and refactor client usage
to match the new API, including changes to PaymentIntents listing.
Update documentation and plans to reference the new version. Remove
references to the old version from go.mod and go.sum.
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.
