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/errors/structured-errors.ts

@@ -3395,6 +3395,22 @@ const SALARY: Record<string, StructuredErrorEntry> = {
     message_sv: 'Lönekörningen är kopplad till en verifikation och kan inte raderas (BFL 5 kap räkenskapsinformation).',
     message_en: 'Salary run is linked to a journal entry and cannot be deleted (BFL 5 kap räkenskapsinformation).',
   },
+  // Utlägg repaid with the salary (#2331).
+  SALARY_RUN_NO_OPEN_EXPENSE_CLAIMS: {
+    httpStatus: 404,
+    message_sv: 'Den anställda har inga öppna utlägg att lägga till.',
+    message_en: 'The employee has no open expense claims to add.',
+  },
+  EXPENSE_CLAIM_ALREADY_ON_PAYSLIP: {
+    httpStatus: 409,
+    message_sv: 'Utlägget ligger redan på ett lönebesked.',
+    message_en: 'The expense claim is already on a payslip.',
+  },
+  SALARY_RUN_EXPENSE_CLAIM_NOT_OPEN: {
+    httpStatus: 409,
+    message_sv: 'Ett utlägg på lönebeskedet är inte längre öppet (utbetalt eller borttaget). Ta bort raden och beräkna om innan bokföring.',
+    message_en: 'An expense claim on the payslip is no longer open (paid or removed). Remove the line and recalculate before booking.',
+  },
   // Phase 5 PR-3: additional import error codes.
   SIE_IMPORT_DUPLICATE: {
     httpStatus: 409,