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>

lib/errors/structured-errors.ts

@@ -2138,6 +2138,20 @@ const SUPPLIER_INVOICE_WAVE4: Record<string, StructuredErrorEntry> = {
     message_en:
       'Periodisering cannot be combined with reverse charge. The expense line carries the VAT base for the VAT declaration (boxes 20-32), so the net amount cannot be deferred to an interim account.',
   },
+  SI_CREATE_SLP_INVALID_ACCOUNT: {
+    httpStatus: 400,
+    message_sv:
+      'Särskild löneskatt kan bara läggas till på rader med pensionskonto 7410-7419 (t.ex. 7412 Premier för tjänstepensioner). Byt konto på raden eller ta bort löneskatten.',
+    message_en:
+      'Särskild löneskatt (payroll tax on pension costs) can only be added on lines booked to a pension account 7410-7419 (e.g. 7412 occupational pension premiums). Change the line account or remove the flag.',
+  },
+  SI_CREATE_SLP_ACCRUAL: {
+    httpStatus: 400,
+    message_sv:
+      'Särskild löneskatt kan inte kombineras med periodisering på samma rad. Löneskatten (7533/2514) beräknas på hela radbeloppet vid registrering och kan inte skjutas upp.',
+    message_en:
+      'Särskild löneskatt cannot be combined with periodisering on the same line. The payroll tax (7533/2514) is computed on the full line amount at registration and cannot be deferred.',
+  },
   SI_DELETE_HAS_BOOKING: {
     httpStatus: 400,
     message_sv: