Handle inventory errors and improve checkout process

[seer-by-sentry]

Fixes REACT-5JE. The issue was that: Inventory check failed in process_order due to insufficient stock (initial stock 1), raising an unhandled exception causing a 500 error.

• Added a custom InventoryError exception to handle out-of-stock situations with specific item IDs.
• Modified process_order to raise InventoryError when insufficient inventory is available, including the requested and available quantities.
• Updated the /checkout route to catch InventoryError and return a 400 error with a JSON response containing the error message and item ID.
• Added more robust error handling in /checkout to catch generic exceptions, report them to Sentry, and return a 500 error.
• Modified the inventory update logic to correctly decrement the inventory based on the quantity requested in the cart.
• Added a copy of the Inventory object to avoid race conditions.

This fix was generated by Seer in Sentry, triggered by [email protected]. 👁️ Run ID: 112266

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To fix the issue, the error message returned to the user should be generic and avoid exposing internal application state. The detailed error message from the InventoryError exception can be logged for debugging purposes, but the response to the user should only indicate that there was an issue with their request.

The best way to implement this fix is to replace the str(ie) usage with a generic error message, such as "Insufficient inventory for the requested item." Additionally, the detailed error message can be logged using sentry_sdk.capture_exception(ie) or another logging mechanism.