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>

extensions/general/invoice-inbox/index.ts

@@ -44,6 +44,7 @@ import {
 import { createSupplierInvoiceRegistrationEntry } from '@/lib/bookkeeping/supplier-invoice-entries'
 import { createSchedulesForSupplierInvoice } from '@/lib/bookkeeping/accruals/from-invoices'
 import { suggestBalanceAccount } from '@/lib/bookkeeping/accruals/account-suggestions'
+import { isSlpPensionAccount } from '@/lib/bookkeeping/slp-lines'
 import { createJournalEntry } from '@/lib/bookkeeping/engine'
 import { bookkeepingErrorResponse } from '@/lib/bookkeeping/errors'
 import { errorResponseFromCode } from '@/lib/errors/get-structured-error'
@@ -1754,6 +1755,28 @@ export const invoiceInboxExtension: Extension = {
           return NextResponse.json({ error: 'Supplier not found' }, { status: 404 })
         }
 
+        // Särskild löneskatt (SLP): same guards as /api/supplier-invoices.
+        // The 7533/2514 pair is only lawful on 741x pension premiums and
+        // cannot be combined with periodisering on the same row.
+        if (
+          body.items.some(
+            (bodyItem) => bodyItem.apply_slp && !isSlpPensionAccount(bodyItem.account_number),
+          )
+        ) {
+          return errorResponseFromCode('SI_CREATE_SLP_INVALID_ACCOUNT', ctx.log)
+        }
+        if (
+          body.items.some(
+            (bodyItem) =>
+              bodyItem.apply_slp &&
+              (bodyItem.accrual_period_start ||
+                bodyItem.accrual_period_end ||
+                bodyItem.accrual_balance_account),
+          )
+        ) {
+          return errorResponseFromCode('SI_CREATE_SLP_ACCRUAL', ctx.log)
+        }
+
         // Periodisering requires faktureringsmetoden: mirror the main
         // /api/supplier-invoices guard so kontantmetod companies never store
         // accrual fields the booking would silently ignore.
@@ -1840,6 +1863,9 @@ export const invoiceInboxExtension: Extension = {
                 ? (bodyItem.accrual_balance_account ??
                   suggestBalanceAccount('expense', bodyItem.account_number))
                 : null,
+            // Särskild löneskatt (SLP): booking injects the self-balancing
+            // 7533/2514 pair for this line. Guarded above (741x, no accrual).
+            apply_slp: bodyItem.apply_slp === true,
           }
         })