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>

types/index.ts

@@ -1267,6 +1267,11 @@ export interface Invoice {
   // Reference
   your_reference: string | null
   our_reference: string | null
+  // Fakturamärkning: buyer-required marking (cost center, project, PO label),
+  // separate from your_reference (Er referens = contact person). Printed on
+  // the PDF and mapped to Peppol BT-10 BuyerReference when set. Optional in
+  // TS for pre-migration fixtures.
+  invoice_marking?: string | null
 
   // Optional online payment link (pasted by the user, e.g. a Stripe Payment
   // Link). Rendered as a "Betala online" button in the invoice email and as a
@@ -1431,7 +1436,12 @@ export interface InvoiceItem {
   // Price
   unit_price: number
 
-  // Calculated
+  // Percentage discount on the line (0-100). line_total and vat_amount are
+  // stored NET of this discount (lib/invoices/line-amounts.ts). Optional in
+  // TS for pre-migration fixtures; treat undefined the same as 0.
+  discount_percent?: number
+
+  // Calculated (net of discount_percent)
   line_total: number
 
   // Per-line VAT
@@ -1708,6 +1718,8 @@ export interface CreateInvoiceInput {
   document_type?: InvoiceDocumentType
   your_reference?: string
   our_reference?: string
+  /** Fakturamärkning: buyer-required marking, separate from your_reference. */
+  invoice_marking?: string
   notes?: string
   /** Optional https link where the customer can pay online (e.g. a Stripe Payment Link). */
   payment_link_url?: string
@@ -1731,6 +1743,8 @@ export interface CreateInvoiceItemInput {
   quantity: number
   unit: string
   unit_price: number
+  /** Percentage discount on the line (0-100). Omitted/null = 0. */
+  discount_percent?: number | null
   vat_rate?: number
   /** Source article (optional). Free-text lines omit it. */
   article_id?: string | null