Fix Issue 875

[razeprasine]

Description
Closes #875

Changes proposed
What were you told to do?
feat(routes-f): invoice total calculator — Compute invoice totals from line items with tax and discount.

What did you do?
Created a new route app/api/routes-f/invoice-total/ with:

• route.ts — POST handler accepting { items: [{ description, qty, unit_price }], tax_percent?, discount_percent? }. Returns { subtotal, discount, taxable, tax, total, line_totals } with cent precision (2 decimal rounding at every step). Uses zod validation via the shared validateBody helper.
• tests/route.test.ts — 8 unit tests covering single line item, multi-line, discount-only, tax-only, tax + discount stacking, multi-line stacking, zero percent edge case, and cent precision rounding.
  All code lives inside app/api/routes-f/invoice-total/ — no files outside that folder were modified.
  Check List
• My code follows the code style of this project.
• This PR does not contain plagiarized content.
• The title and description of the PR is clear and explains the approach.
• I am making a pull request against the main branch (left side).
• My commit messages styles matches our requested structure.
• My code additions will fail neither code linting checks nor unit test.
• I am only making changes to files I was requested to.
  Screenshots/Videos
  N/A — pure backend route, no UI changes.

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