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/__tests__/services/rasterizePageCap.test.js

@@ -0,0 +1,10 @@
+const rasterizeService = require('../../src/services/rasterizeService');
+
+// The page-range guard runs BEFORE any fs/pdftoppm work, so these reject
+// without touching the binary or disk (PR #622 concern 6).
+describe('getRenderedPagePath page-range guard', () => {
+  it.each([0, -1, 201, 1000, 1.5, NaN])('rejects out-of-range page %p', async (page) => {
+    await expect(rasterizeService.getRenderedPagePath(1, '/tmp/does-not-exist.pdf', page))
+      .rejects.toMatchObject({ statusCode: 400, code: 'PAGE_OUT_OF_RANGE' });
+  });
+});