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>

app/api/v1/companies/[companyId]/invoices/[id]/credit/route.ts

@@ -39,7 +39,7 @@ const CreditNoteRequest = z.object({
 })
 
 const ORIGINAL_INVOICE_COLUMNS =
-  'id, invoice_number, customer_id, invoice_date, due_date, delivery_date, status, currency, exchange_rate, exchange_rate_date, subtotal, subtotal_sek, vat_amount, vat_amount_sek, total, total_sek, vat_treatment, vat_rate, moms_ruta, your_reference, our_reference, notes, reverse_charge_text, credited_invoice_id, document_type, default_dimensions'
+  'id, invoice_number, customer_id, invoice_date, due_date, delivery_date, status, currency, exchange_rate, exchange_rate_date, subtotal, subtotal_sek, vat_amount, vat_amount_sek, total, total_sek, vat_treatment, vat_rate, moms_ruta, your_reference, our_reference, invoice_marking, notes, reverse_charge_text, credited_invoice_id, document_type, default_dimensions'
 
 // default_dimensions stays in this projection: the inserted credit-note row is
 // handed to createCreditNoteJournalEntry, which reads the bag off the row so
@@ -48,7 +48,7 @@ const CREDIT_NOTE_RESPONSE_COLUMNS =
   'id, invoice_number, customer_id, invoice_date, due_date, delivery_date, status, currency, exchange_rate, exchange_rate_date, subtotal, subtotal_sek, vat_amount, vat_amount_sek, total, total_sek, vat_treatment, vat_rate, moms_ruta, your_reference, our_reference, notes, reverse_charge_text, credited_invoice_id, document_type, paid_at, paid_amount, remaining_amount, default_dimensions, created_at, updated_at'
 
 const ORIGINAL_ITEMS_COLUMNS =
-  'sort_order, description, quantity, unit, unit_price, line_total, vat_rate, vat_amount, dimensions'
+  'sort_order, description, quantity, unit, unit_price, discount_percent, line_total, vat_rate, vat_amount, dimensions'
 
 const CreditNoteCreated = z.object({
   id: z.string().uuid(),
@@ -181,6 +181,7 @@ export const POST = withApiV1<{ params: Promise<{ companyId: string; id: string
         quantity: number
         unit: string
         unit_price: number
+        discount_percent?: number | null
         line_total: number
         vat_rate?: number | null
         vat_amount?: number | null
@@ -246,6 +247,8 @@ export const POST = withApiV1<{ params: Promise<{ companyId: string; id: string
       reverse_charge_text: original.reverse_charge_text ?? null,
       your_reference: original.your_reference ?? null,
       our_reference: original.our_reference ?? null,
+      // Same buyer routing on the kreditfaktura as the original.
+      invoice_marking: original.invoice_marking ?? null,
       notes: reason || `Krediterar faktura ${original.invoice_number ?? original.id}`,
       credited_invoice_id: originalId,
       status: 'sent' as const,
@@ -261,6 +264,9 @@ export const POST = withApiV1<{ params: Promise<{ companyId: string; id: string
       quantity: -Math.abs(item.quantity),
       unit: item.unit,
       unit_price: item.unit_price,
+      // Carried so the kreditfaktura's face arithmetic multiplies out and the
+      // Rabatt column renders (ML 17 kap 24 §).
+      discount_percent: item.discount_percent ?? 0,
       line_total: -Math.abs(item.line_total),
       vat_rate: item.vat_rate ?? 0,
       vat_amount: -Math.abs(item.vat_amount ?? 0),