feat(accounting): Layer A backend — chart of accounts, VAT codes, Treuhänder export

Prepares picpeak to feed a Treuhänder's double-entry software once a user
crosses the CHF ~500k threshold (LI PGR Art. 1045), without becoming an ERP.

- migration 129: ledger_accounts (seeded Swiss/LI KMU-Kontenrahmen) +
  vat_codes (CH/LI MWST 8.1/2.6/3.8/0 + reverse charge), expense_categories
  gains ledger_account_id, app_settings default-account + VAT-map seeds
- ledgerService: full CRUD for accounts + VAT codes + mappings; buildPostings()
  turns revenue invoices + incoming invoices + expenses into accrual
  Buchungssätze (Dr/Cr + VAT code); generic/banana/bexio CSV export
- routes /api/admin/ledger/* (accounting master gated; export also requires
  taxReport); 12 unit tests (posting engine + formatters)

Accrual basis only — payment/bank postings are Layer B. Output is a guideline
(Treuhänder caveat on the UI).

backend/server.js

@@ -707,6 +707,7 @@ app.use('/api/admin/calendar',   require('./src/routes/adminCalendar'));
 app.use('/api/admin/deals',      require('./src/routes/adminDeals'));
 app.use('/api/admin/tax-report', require('./src/routes/adminTaxReport'));
 app.use('/api/admin/expenses',   require('./src/routes/adminExpenses'));
+app.use('/api/admin/ledger',     require('./src/routes/adminLedger'));
 app.use('/api/admin/system-health', require('./src/routes/adminSystemHealth'));
 app.use('/api/admin/dev',        require('./src/routes/adminDev'));
 app.use('/api/public/quotes',  require('./src/routes/publicQuotes'));