feat(invoices): per-line percentage discount and separate fakturamarkning (#2084)

* feat(invoices): per-line percentage discount and separate fakturamarkning

User request: rabatt i procent per artikelrad, and a marking field
separate from Er referens.

- invoice_items.discount_percent (0-100, default 0): line_total and
  vat_amount are stored NET of the discount. Shared exact-ore math in
  lib/invoices/line-amounts.ts (gross, discount, net) used by the web
  builder, staged-operation commit, editor preview, PDF, and Peppol.
  Undiscounted lines keep the legacy unrounded qty*price byte-identical.
- ROT/RUT deduction computes on the discounted net line total.
- invoices.invoice_marking: printed on the PDF next to the references
  and mapped to Peppol BT-10 BuyerReference (marking wins over
  your_reference; either satisfies the BT-10 requirement).
- Peppol renders the discount as a BG-27 line AllowanceCharge
  (reason code 95, MultiplierFactorNumeric, Amount, BaseAmount).
- Editor: "Lagg till rabatt" in the row menu (same reveal pattern as
  ROT/RUT), Markning row next to Er referens, forval chip, review
  dialog shows discounts and marking.
- Plumbed through v1 REST projections, MCP create/get/update invoice
  tools, pending-operations update path, and copy-invoice (discount
  copied; marking deliberately not, it is recipient-specific).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JJAt9yM7tgZ69f1XnNnq52

* fix(invoices): carry discount_percent through every deduction, credit, convert and preview path

Skeptic + CI findings on the discount/marking feature, one pass:

- generateRotRutLines and propose-send-lines now pass discount_percent
  into computeDeduction: the send/credit/cash verifikat booked 1513 on
  the GROSS line while deduction_total, the PDF and the Skatteverket
  claim carried the net, stranding the difference on 1513 and pushing
  1510 negative once the customer paid. Test pins 1513=3000/1510=7000
  for a 20%-discounted 10 000 kr ROT line.
- preview-pdf route accepts discount_percent (net totals + net-based
  deduction) and invoice_marking; the editor now sends the marking, so
  the preview equals the invoice it becomes.
- Credit notes carry discount_percent (buildCreditNoteItem, v1 credit
  route select+insert, MCP credit executor) and invoice_marking, so the
  kreditfaktura face arithmetic multiplies out and shows the Rabatt
  column (ML 17 kap 24 §).
- Proforma->invoice convert copies discount_percent + invoice_marking:
  the converted invoice previously failed Peppol LINE_TOTAL_MISMATCH
  and lost the rebate on the next builder pass.
- Editor hides the discount menu in self-billed mode (the self-billed
  wire shape has no discount; previewed net would book gross).
- MCP staging and commitCreateInvoice reject a non-number
  discount_percent (a string coerced past the range check but was
  ignored by the totals math and still stored).
- Regenerated skills/accounted-api (apiskill:check CI failure).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JJAt9yM7tgZ69f1XnNnq52

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

lib/bookkeeping/__tests__/invoice-entries.test.ts

@@ -1316,6 +1316,44 @@ describe('createInvoiceJournalEntry: ROT/RUT-avdrag', () => {
     expect(totalDebit).toBe(12500)
   })
 
+  it('discounted ROT line: 1513 books the NET-based deduction, matching the stored deduction_total', async () => {
+    // 10 000 kr labor, 20% rabatt → net 8 000 + 25% VAT 2 000 = 10 000.
+    // ROT = 30% of the NET inkl.-moms labor = 30% of 10 000 = 3 000, the same
+    // figure build-invoice-write stores on deduction_total and the payout
+    // request claims. Booking the gross (3 750) would strand 750 kr on 1513
+    // and push kundfordringar (1510) negative when the customer pays 7 000.
+    const invoice = makeInvoice({
+      subtotal: 8000,
+      vat_amount: 2000,
+      total: 10000,
+      vat_treatment: 'standard_25',
+      deduction_total: 3000,
+      items: [
+        makeItem({
+          quantity: 1,
+          unit_price: 10000,
+          discount_percent: 20,
+          line_total: 8000,
+          vat_rate: 25,
+          vat_amount: 2000,
+          deduction_type: 'rot',
+          deduction_amount: 3000,
+        }),
+      ],
+    })
+
+    await createInvoiceJournalEntry(null as never, 'company-1', 'user-1', invoice)
+
+    const input = mockedCreateEntry.mock.calls[0][3]
+    const debit1513 = input.lines.find((l) => l.account_number === '1513')
+    expect(debit1513?.debit_amount).toBe(3000)
+    const debit1510 = input.lines.find((l) => l.account_number === '1510')
+    expect(debit1510?.debit_amount).toBe(7000)
+    const totalDebit = input.lines.reduce((sum, l) => sum + l.debit_amount, 0)
+    const totalCredit = input.lines.reduce((sum, l) => sum + l.credit_amount, 0)
+    expect(totalDebit).toBe(totalCredit)
+  })
+
   it('mixed invoice: ROT line + non-deduction line, per-item handling', async () => {
     // ROT line 10 000 (deduction 30% of 12 500 inkl. moms = 3 750) +
     // non-deduction materials line 4 000.

lib/bookkeeping/invoice-entries.ts

@@ -331,6 +331,10 @@ function generateRotRutLines(
     const amount = computeDeduction({
       unit_price: side === 'credit' ? Math.abs(item.unit_price) : item.unit_price,
       quantity: side === 'credit' ? Math.abs(item.quantity) : item.quantity,
+      // The deduction base is the NET line total (rabatt reduces what the
+      // customer pays); omitting this books 1513 on the gross while the
+      // stored deduction_total and the Skatteverket claim carry the net.
+      discount_percent: item.discount_percent ?? 0,
       deduction_type: item.deduction_type,
       vat_rate: item.vat_rate,
     })

lib/bookkeeping/propose-send-lines.ts

@@ -245,6 +245,9 @@ function buildSendLines(
     const deduction = computeDeduction({
       unit_price: item.unit_price,
       quantity: item.quantity,
+      // Net of any line discount: must match the stored deduction_total or
+      // the proposed 1513/1510 split cannot clear.
+      discount_percent: item.discount_percent ?? 0,
       deduction_type: item.deduction_type,
       vat_rate: item.vat_rate,
     })