Title: Fix order total rounding error

Classification: type=Task, area=backend, execution_order=follow-up, dependency_level=unblocked, target_bucket=ready-to-refine

Description: When applying percentage discounts to orders with multiple line items, totals are rounding inconsistently causing a mismatch between displayed total and sum of line items.

Steps to reproduce:

1. Create cart with items: A (price 199.99, qty 1), B (price 49.50, qty 2)
2. Apply discount code 'MULTI5' (5%)
3. Place order and observe total displayed vs sum of line items

Acceptance criteria:

• Add unit test reproducing mismatch
• Fix rounding logic so displayed total equals sum of adjusted line items
• Add regression spec

Estimate: 2

Labels: bug, billing

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