feat(routes-f): invoice total calculator

[davedumto]

Summary

Compute invoice totals from line items with tax and discount.

Requirements

• POST { items: [{ description, qty, unit_price }], tax_percent?, discount_percent? }
• Return { subtotal, discount, taxable, tax, total, line_totals }
• Cent precision
• Tests cover multi-line, discount, tax stacking

Scope Constraint - READ BEFORE STARTING

All files for this task must live inside app/api/routes-f/. This includes the route handler, helpers/utilities, types, and tests. Do NOT modify, import from, or add files to lib/, utils/, types/, components/, or anywhere else outside app/api/routes-f/. If you need shared logic, duplicate it inside your subfolder. Keeping everything scoped to this folder is intentional - it keeps these tasks independent and mergeable in any order.

[razeprasine]

@razeprasine has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hi maintainer, I will love to work on this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @razeprasine to this issue.

[drips-wave]

Congratulations, @razeprasine! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 2, 2026.

🧑‍💻 @razeprasine: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

Warning

When opening a PR, please link it to this issue to ensure it gets tracked accurately. Points are awarded when this issue is marked as completed by the maintainer.

🤠 Repo maintainers: Please keep an eye on the contributor's progress and review their work before the due date. You can manage this issue, including adjusting its complexity and points, here.

🌊 Happy Wave 🌊