Refactor process_order to handle quantities and string item IDs

[seer-by-sentry]

Fixes ANDROID-25. The issue was that: Backend inventory shortage caused checkout POST 500, triggering Android error handling that intentionally throws 'Failed to init delivery workflow' exception.

• Increased initial inventory for all items to 10.
• Added item '3' to the inventory with a quantity of 10.
• Modified process_order to accept a cart dictionary containing 'items' and 'quantities'.
• Converted item IDs to strings for consistency.
• Implemented quantity handling in process_order, allowing for purchasing multiple items at once.
• Updated inventory update logic to reflect purchased quantities.

This fix was generated by Seer in Sentry, triggered by Dustin Bailey. 👁️ Run ID: 49403