Fix/issue 2342 atomic checkout

[Prateek2007-cmd]

Description

Resolves Issue #2342 (Lack of Atomic Stock Verification).

The checkout logic previously lacked any backend validation of inventory, exposing the system to severe race conditions where overselling limited-stock items was guaranteed under moderate load.

This PR introduces a robust, atomic backend verification layer.

Changes Made

• Inventory Tracking: Extended the Product SQLAlchemy model with a stock column.
• Row-Level Locking: Implemented a POST /api/products/checkout endpoint that enforces transactional atomic locks (.with_for_update()) on product rows during checkout processing.
• Race Condition Prevention: The system now guarantees stock is available and safely decrements it within a single atomic database commit, rejecting the order with a 400 Bad Request if stock falls below the requested quantity.
• Frontend Sync: Upgraded checkout.js to dispatch authentic checkout payloads instead of simulating success.

Type of Change

• Architectural Overhaul
• Bug Fix

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