test(accounting): cover export scope, unique-violation detector, PDF page cap

Closes the test gaps from the PR #622 work + the export-scope feature:
- export scope: scopeLedger/normalizeScope (exported via _internal) unit tests +
  renderTaxReportCsv income/cost/all output assertions (income drops supplier
  rows, cost drops invoice rows, filename gets the scope tag).
- isUniqueViolation: Postgres 23505 / SQLITE_CONSTRAINT / "UNIQUE constraint
  failed" message, false for FK + nullish (the IMAP claim-first race detector).
- getRenderedPagePath: out-of-range pages reject with PAGE_OUT_OF_RANGE before
  touching pdftoppm/disk (the per-file resource bound).

backend/src/services/taxReportService.js

@@ -1121,5 +1121,5 @@ module.exports = {
   renderTaxReportPdf,
   renderTaxReportCsv,
   // Exposed for unit tests.
-  _internal: { grossUpLateFee, computeReportedAmounts, buildCustomerLabel, formatVatRate, loadCosts },
+  _internal: { grossUpLateFee, computeReportedAmounts, buildCustomerLabel, formatVatRate, loadCosts, scopeLedger, normalizeScope },
 };