feat(supplier-invoices): sarskild loneskatt (SLP) pair on pension premium lines (#1534)

* feat(supplier-invoices): sarskild loneskatt (SLP) pair on pension premium lines

Booking a tjanstepension invoice (e.g. Avanza) needs the buyer's own SLP
beyond the payable: debit 7533 / credit 2514 at 24.26% of the premium
(SLF 1991:687). The item-based debit-only form could not express the
self-balancing pair, so users had to hand-edit the verifikat.

- new leaf module lib/bookkeeping/slp-lines.ts: SLP_RATE (single source,
  re-exported by the bokslut calculator), isSlpPensionAccount (741x),
  generateSlpLines (7533 D / 2514 K, nets to zero)
- migration adds supplier_invoice_items.apply_slp boolean default false
- registration, cash, and privately-paid generators inject the pair for
  flagged 741x items, mirroring the reverse-charge injection; the balance
  guarantees keep 2440/1930/2893 at exactly the invoice total; the credit
  note generator reverses the pair (7533 K / 2514 D)
- privately-paid balance guarantee now subtracts existing credits so the
  SLP 2514 leg never inflates the owner account
- schema field apply_slp + guards in all create paths (main route, inbox
  convert, v1 REST, pending-operations executor): 400
  SI_CREATE_SLP_INVALID_ACCOUNT on non-741x accounts, 400
  SI_CREATE_SLP_ACCRUAL combined with periodisering
- form: advisory hint on unflagged 741x rows with one-click opt-in and a
  quiet confirmation line when applied; totals box untouched (the invoice
  total stays the payable); AB review preview injects the same pair via
  the same generator for parity
- year-end double-count guard: calculateSarskildLoneskatt subtracts SLP
  already posted to 7533 during the year (floored at zero) so bokslut
  never provisions flagged premiums twice

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* chore(api-skill): regenerate suppliers reference for apply_slp

The apiskill:check CI gate requires the generated accounted-api skill to
stay in sync with the endpoint registry after the apply_slp addition.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* fix(slp): carry apply_slp through v1 routes, MCP staging, preview and credit reversal

Review findings on the SLP PR:

- v1 credit route: SI_FULL_COLUMNS now projects items.apply_slp, so
  createSupplierCreditNoteEntry sees the flag and reverses the 7533/2514
  pair booked at registration (it previously stood forever and the
  year-end netting under-provisioned). The flag is also copied onto the
  created credit-note items for parity with the web credit route.
- v1 mark-paid: the items sub-select now includes apply_slp, so a
  kontantmetoden payment via v1 books the cash entry WITH the SLP pair,
  matching the web mark-paid.
- v1 GET ?expand=items: SI_ITEM_COLUMNS includes apply_slp so the flag
  is readable back through the public API.
- credit-note SLP base is abs of the SIGNED sum of flagged line_totals,
  not per-item abs: a mixed-sign flagged original (+10000/-2000) booked
  SLP on 8000 at registration and now reverses exactly that, not 12000.
  The expense-bucket per-item abs convention is untouched.
- kontantmetod bank-match preview appends the same generateSlpLines pair
  the POST books, so the approved lines equal the committed lines.
- MCP gnubok_create_supplier_invoice_from_inbox: line_overrides accepts
  apply_slp (optional boolean), plumbs it into the staged operation's
  items, and rejects non-741x resolved accounts at staging time with the
  bilingual SI_CREATE_SLP_INVALID_ACCOUNT texts.
- DECISIONS.md: five entries for today's decisions.

Every behavioral fix has a test verified to fail without it.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Jakob Wennberg <311770904+jakobwennberg-oss@users.noreply.github.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

components/suppliers/SupplierInvoiceReviewContent.tsx

@@ -12,6 +12,7 @@ import {
   generateReverseChargeBasisLines,
   generateReverseChargeLines,
 } from '@/lib/bookkeeping/vat-entries'
+import { generateSlpLines, isSlpPensionAccount } from '@/lib/bookkeeping/slp-lines'
 import { buildSupplierDescription } from '@/lib/bookkeeping/supplier-invoice-description'
 import { resolveBookingAccount, itemHasAccrual } from '@/lib/bookkeeping/accruals/account-suggestions'
 import type { Supplier } from '@/types'
@@ -27,6 +28,9 @@ interface ReviewLineItem {
   // Self-assessed VAT rate for omvänd skattskyldighet (0.06/0.12/0.25). The
   // supplier charges no VAT (vat_rate = 0); this drives the fiktiv-moms preview.
   reverse_charge_rate?: number
+  // Särskild löneskatt på pensionskostnader: previews the self-balancing
+  // 7533 D / 2514 K pair the engine injects for flagged 741x lines.
+  apply_slp?: boolean
   // Periodisering: when both dates are set, the registration entry books the
   // net to the 17xx interim account instead of account_number (mirrored via
   // resolveBookingAccount so this preview matches the saved verifikat).
@@ -114,6 +118,24 @@ function buildJournalPreview(
       ? Math.round(item.vat_amount * 100) / 100
       : Math.round(item.amount * item.vat_rate * 100) / 100
 
+  // Särskild löneskatt på pensionskostnader: same self-balancing pair the
+  // engine injects (7533 D / 2514 K at 24.26 % of flagged 741x lines), via
+  // the same generator, so this preview matches the saved verifikat. The
+  // pair nets to zero and never moves the 2440 credit below.
+  const slpBase = items.reduce(
+    (sum, item) =>
+      item.apply_slp && isSlpPensionAccount(item.account_number)
+        ? sum + toSek(item.amount)
+        : sum,
+    0,
+  )
+  const slpPreviewLines: JournalPreviewLine[] = generateSlpLines(slpBase).map((sl) => ({
+    account_number: sl.account_number,
+    description: sl.line_description ?? sl.account_number,
+    debit: sl.debit_amount,
+    credit: sl.credit_amount,
+  }))
+
   if (reverseCharge) {
     // Reverse charge: the supplier charges no VAT, so the buyer self-assesses at
     // the Swedish statutory rate (resolveReverseChargeRate: 25% huvudregel
@@ -170,6 +192,8 @@ function buildJournalPreview(
       }
     }
 
+    lines.push(...slpPreviewLines)
+
     // Credit: 2440 at subtotal (no real VAT for reverse charge)
     lines.push({
       account_number: '2440',
@@ -197,6 +221,8 @@ function buildJournalPreview(
         })
       }
     }
+    lines.push(...slpPreviewLines)
+
     // Credit: 2440 at total incl. VAT
     lines.push({
       account_number: '2440',