feat(salary): repay utlägg with the salary as a tax-free payslip line (#2361)

* feat(salary): repay utlägg with the salary as a tax-free payslip line (#2331)

- expense_reimbursement line type: kostnadsersättning outside gross, tax,
  avgifter and the AGI. The engine adds tax-free reimbursements (utlägg,
  skattefritt traktamente, skattefri milersättning) to the net payout only.
- booking debits the claim's liability account (2820) on top of gross,
  never a 7xxx cost; a run that only repays utlägg posts 2820 D / 1930 K
  instead of being treated as a nollkörning
- salary_line_items.source_expense_claim_id (tenant-scoped FK, cascade,
  one payslip line per claim); settle_expense_claims_via_salary_run marks
  the claims paid with an expense_payout_batches row pointing at the
  salary verifikat, no second verifikat, idempotent on retry; wired into
  bookLoadedRun and the v1 book route with a pre-check before posting
- create_expense_payout_batch refuses claims scheduled on a payslip
  (ON_PAYSLIP); deleteExpenseClaim refuses once the run has left draft
- "Lägg till utlägg" on the employee row of a draft run; the payslip page
  labels and removes the lines
- pg-real: tests/pg/utlagg-via-lon.pg.test.ts + ON_PAYSLIP case

Claude-Session: https://claude.ai/code/session_01LvMaHcTnwAfxzgYD1fGYX1
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

* fix(salary): PR #2361 review: claim delete cannot cascade into a booked payslip; AGI excludes the utlägg line

- salary_line_items_source_expense_claim_fkey is ON DELETE RESTRICT (edited
  in the unmerged 20260906210300): the database refuses to delete a claim a
  payslip line still references, whichever path issues the DELETE
- deleteExpenseClaim removes the draft line first (before the storno) and
  keeps refusing with ON_PAYSLIP once the run has left draft
- pg-real: delete refused with 23503 on a booked and on a draft run; the
  app order (line, then claim) succeeds
- unit: AGI builder keeps FK011/FK001/FK487 and emits no benefit field for
  an expense_reimbursement line (FK011 derives from sre.gross_salary; only
  benefit_* types are read from line items)

Claude-Session: https://claude.ai/code/session_01LvMaHcTnwAfxzgYD1fGYX1
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

---------

Co-authored-by: Jakob Wennberg <311770904+jakobwennberg-oss@users.noreply.github.com>
Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>

lib/salary/calculation-engine.ts

@@ -2,6 +2,7 @@ import type { PayrollConfig } from './payroll-config'
 import type { TaxTableRate } from './tax-tables'
 import { lookupTaxAmount, calculateJamkningTax, calculateSidoinkomstTax } from './tax-tables'
 import { calculateAgeAtYearStart, decryptPersonnummer } from './personnummer'
+import { TAX_FREE_REIMBURSEMENT_TYPES } from './account-mapping'
 import type { SalaryLineItemType } from '@/types'
 
 // ============================================================
@@ -96,6 +97,12 @@ export interface SalaryCalculationResult {
   taxableIncome: number
   taxWithheld: number
   netDeductions: number
+  /**
+   * Kostnadsersättning paid out with the salary (utlägg, skattefritt
+   * traktamente, skattefri milersättning). Inside netSalary, outside
+   * grossSalary, taxableIncome and avgifterBasis.
+   */
+  taxFreeReimbursements: number
   netSalary: number
   /** Öre added to reach a whole-krona net payout (0 when rounding is off or the net is already whole). */
   netRounding: number
@@ -484,6 +491,26 @@ export function calculateSalary(
     output: netSalary,
   })
 
+  // ─── Step 7a: Kostnadsersättning (skattefri) ───
+  // Utlägg, skattefritt traktamente and skattefri milersättning are paid out
+  // with the salary but are not lön: no skatteavdrag, no arbetsgivaravgifter,
+  // no semesterunderlag, not in the AGI gross (travel-expenses.md). They ride
+  // on the payout only, after tax and net deductions and before the
+  // öresavrundning of the final amount.
+  const reimbursementItems = input.lineItems.filter(
+    li => TAX_FREE_REIMBURSEMENT_TYPES.includes(li.itemType) && li.amount > 0
+  )
+  const taxFreeReimbursements = r(reimbursementItems.reduce((sum, li) => sum + li.amount, 0))
+  if (taxFreeReimbursements > 0) {
+    netSalary = r(netSalary + taxFreeReimbursements)
+    steps.push({
+      label: 'Kostnadsersättning (skattefri)',
+      formula: 'nettolön + skattefria ersättningar (utlägg, traktamente, milersättning)',
+      input: { count: reimbursementItems.length, reimbursements: taxFreeReimbursements },
+      output: netSalary,
+    })
+  }
+
   // ─── Step 7b: Öresavrundning (optional, uppåt till hel krona) ───
   // Integer öre arithmetic: netSalary is already r()-rounded so netOre is
   // exact; ceil-by-remainder avoids float noise. Only positive payouts round:
@@ -642,6 +669,7 @@ export function calculateSalary(
     taxableIncome,
     taxWithheld,
     netDeductions: totalNetDeductions,
+    taxFreeReimbursements,
     netSalary,
     netRounding,
     avgifterRate: avgifterCalc.rate,