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>

messages/en.json

@@ -4165,6 +4165,10 @@
     "illegal_vat_rate_description": "Line {row} has VAT rate {rate} %. The legal Swedish VAT rates are 25, 12, 6 or 0 %.",
     "rc_account_warning": "Reverse charge: {count, plural, =1 {line {rows} uses an account starting with 1 or 6} other {lines {rows} use accounts starting with 1 or 6}}. Reverse charge purchases are normally booked on cost accounts (4xxx/5xxx). Double-check the account choice.",
     "foreign_zero_vat_warning": "Foreign supplier with no Swedish VAT: {count, plural, =1 {line {rows} has} other {lines {rows} have}} 0 % VAT while reverse charge is switched off. For a service bought from abroad, reverse charge should be on; otherwise boxes 20-24, 30-32 and 48 stay empty in the VAT return. If the supplier charged its own local VAT instead, or this is a goods import handled by customs, 0 % is correct and you can ignore this.",
+    "slp_hint": "Pension insurance: add särskild löneskatt (24.26 %), {amount} kr. The tax is booked as its own line pair (7533/2514) and does not change the invoice total.",
+    "slp_add_action": "Add payroll tax",
+    "slp_applied_line": "7533 Särskild löneskatt D {amount} / 2514 C {amount}, does not affect the invoice total",
+    "slp_remove_action": "Remove",
     "expense_registered_title": "Expense registered",
     "invoice_registered_title": "Invoice registered",
     "arrival_number_label": "Arrival number: {number}",

messages/sv.json

@@ -4165,6 +4165,10 @@
     "illegal_vat_rate_description": "Rad {row} har momssats {rate} %. Tillåtna momssatser är 25, 12, 6 eller 0 %.",
     "rc_account_warning": "Omvänd skattskyldighet: {count, plural, =1 {rad {rows} använder ett konto som börjar på 1 eller 6} other {raderna {rows} använder konton som börjar på 1 eller 6}}. Inköp med omvänd skattskyldighet bokförs normalt på kostnadskonton (4xxx/5xxx). Kontrollera kontovalet.",
     "foreign_zero_vat_warning": "Utländsk leverantör utan svensk moms: {count, plural, =1 {rad {rows} har} other {raderna {rows} har}} 0 % moms medan omvänd skattskyldighet är avstängd. Är det ett tjänsteköp från utlandet ska omvänd skattskyldighet vara på, annars blir ruta 20-24, 30-32 och 48 tomma i momsdeklarationen. Har leverantören i stället debiterat sin egen utländska moms, eller är det en varuimport som tullen hanterar, är 0 % rätt och du kan bortse från detta.",
+    "slp_hint": "Pensionsförsäkring: lägg till särskild löneskatt (24,26 %), {amount} kr. Skatten bokförs som ett eget radpar (7533/2514) och påverkar inte fakturabeloppet.",
+    "slp_add_action": "Lägg till löneskatt",
+    "slp_applied_line": "7533 Särskild löneskatt D {amount} / 2514 K {amount}, påverkar inte fakturabeloppet",
+    "slp_remove_action": "Ta bort",
     "expense_registered_title": "Utlägg registrerat",
     "invoice_registered_title": "Faktura registrerad",
     "arrival_number_label": "Ankomstnummer: {number}",