bug: inclusive-tax items produce inflated total when order-level discount is applied

[khaira777]

Bug Description

When a product is configured with tax_type = 'inclusive' (price already contains tax) and an order-level discount is applied, the order total is incorrectly inflated. The code adds the extracted tax component on top of a discounted subtotal that already includes the tax inside it.

Steps to Reproduce

1. Create a product with tax_type = 'inclusive', tax_rate = 5, price = ₹1000
2. Create an order with this product
3. Apply a 10% order-level discount via PATCH /orders/:id/discount
4. Observe the resulting total

Expected vs Actual

	Value
Discounted subtotal	₹900
Tax (inclusive — already inside the price)	embedded in ₹900
Expected total	₹900
Scaled tax incorrectly added on top (bug)	₹42.86
Actual total	₹942.86 ❌

Root Cause

In main/routes/orders.ts, when a discount is applied the code:

1. Computes discountedSubtotal = subtotal - discountAmount
2. Scales newTaxAmount = freshTax × (discountedSubtotal / subtotal)
3. Computes total = discountedSubtotal + newTaxAmount

For exclusive tax this is correct — tax is separate from the price.
For inclusive tax, subtotal already contains the tax. After discounting, discountedSubtotal still contains the embedded tax — adding newTaxAmount on top is double-counting.

Affected Code

• main/routes/orders.ts — PATCH /:id/discount handler (~line 695)
• main/routes/orders.ts — POST /:id/items add-items recalculation (~line 396)
• main/routes/orders.ts — PATCH /:id/items/:itemId/discount order recalculation (~line 884)

Suggested Fix

The ratio approach needs to be aware of tax type. Options:

Option A (simpler): Separate the order subtotal into inclusive and exclusive portions before applying the ratio. Only add newTaxAmount to the exclusive portion; the inclusive portion needs no additional tax added.

Option B (more correct): Rather than scaling the original freshTax by ratio at the order level, recalculate item-level tax properly by running computeTaxAmount('inclusive', discountedItemSubtotal, rate) per item after the discount is applied — matching how item creation works.

Impact

• Only affects orders with tax_type = 'inclusive' products + an order-level discount applied simultaneously
• Without a discount: inclusive tax is calculated correctly ✅
• Exclusive tax at any time: always correct ✅
• Common in Southeast Asia (Thailand VAT), Europe, and UK where tax-inclusive pricing is standard

Not Introduced By Recent Commits

This is a pre-existing bug. The recent 4aee1ce commit correctly fixed the tax compounding issue (closed #60) but the inclusive-tax + discount interaction was never handled in any prior commit.