fix: add stock validation and decrement on order placementAdd stock validation and decrement on orders

[AaravKashyap12]

Closes #12

What this PR does

Fixes two related inventory bugs in server/routes/orders.js:

1. Stock validation before order is accepted:

if (product.stock < item.quantity) {
    return res.status(400).json({
        message: `Insufficient stock for "${product.name}". Available: ${product.stock}`
    });
}

2. Atomic stock decrement after order is saved:

for (const item of items) {
    await Product.findByIdAndUpdate(item.product, {
        $inc: { stock: -item.quantity }
    });
}

Why this matters

Without this fix, customers can order any quantity regardless of available stock, and inventory is never updated — making the stock field meaningless.

Changes

File	Change
server/routes/orders.js	Stock check + decrement logic
server/test/orders.test.js	2 regression tests (insufficient stock, valid order)
server/package.json	Added test script

Verification

npm test → 2 passed, 0 failing

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