fix(salary): stop RLS from failing vab/parental absence registration (#1568)

Migration 20260517135000 rewrote the franvaro-specifikationsnummer trigger
functions to insert audit rows into salary_absence_franvaro_audit, a table
with RLS enabled and zero policies, while leaving the functions SECURITY
INVOKER (its comment claimed implicit SECURITY DEFINER, which is false in
Postgres). Every vab/parental insert from role authenticated (dashboard
absence POST, web /pending approval, in-app Assistenten chat) then failed
with 42501, surfaced as a generic 500, and left no diagnosable trace.

- New migration 20260813120000: ALTER both trigger functions to SECURITY
  DEFINER with search_path pinned to public, pg_temp. No RLS policy is added
  on the audit table: trigger/service-only writes stay the design intent.
- mapInsertError: 42501 now maps to the new bilingual DB_PERMISSION_DENIED
  code instead of INTERNAL_ERROR, and 23514 is split so only the 24h-cap
  trigger's 'Total tid' message becomes ABSENCE_HOURS_CONFLICT; other CHECK
  violations map to VALIDATION_ERROR.
- commitRegisterAbsence/commitDeleteAbsence: log the underlying PG details
  and persist the sanitized structured code in result_data.error_code so the
  next failure is traceable from the op row.
- Dashboard absence route: only ABSENCE_HOURS_CONFLICT passes details.message
  through to the client; every other code shows the registry Swedish message
  instead of raw Postgres text.
- New pg-real regression test locks the authenticated-role parental/vab
  insert path, the shared per-month specnummer sequence, the audit rows, and
  idempotent upsert retries.

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

@@ -81,6 +81,16 @@ const GENERIC: Record<string, StructuredErrorEntry> = {
     message_sv: 'Du har inte behörighet att utföra denna åtgärd.',
     message_en: 'Insufficient permissions.',
   },
+  // A Postgres privilege/RLS denial (42501) on a write the application
+  // expected to succeed: a server-side configuration bug (e.g. a SECURITY
+  // INVOKER trigger writing to a policy-less RLS table), not a user-permission
+  // problem. Kept distinct from FORBIDDEN (which blames the user) and from
+  // INTERNAL_ERROR (which hides the failure mode from diagnostics).
+  DB_PERMISSION_DENIED: {
+    httpStatus: 500,
+    message_sv: 'Ett behörighetsfel i databasen stoppade åtgärden. Kontakta supporten om felet kvarstår.',
+    message_en: 'A database permission (RLS) denial blocked the write. This indicates a server-side misconfiguration.',
+  },
   NOT_FOUND: {
     httpStatus: 404,
     message_sv: 'Resursen kunde inte hittas.',