feat(api): [AB#151] QuickBooks data sync — clients, invoices, and payments

[bllr777]

Changes

• Add \IQuickBooksSyncService\ interface to Business layer
• Implement \QuickBooksSyncService\ in Infrastructure (QB Accounting REST API)
  • \SyncClientAsync\ — create or sparse-update QB Customer; stores \QuickBooksCustomerId\ back on \OrganizationClient\
  • \SyncInvoiceAsync\ — create or sparse-update QB Invoice (line items mapped); stores \QuickBooksInvoiceId\ back on \Invoice; auto-syncs client first if not yet synced
  • \SyncPaymentAsync\ — create QB Payment linked to the QB Invoice
• Add \QuickBooksCustomerId\ to \OrganizationClient, \QuickBooksInvoiceId\ to \Invoice\
• Migration: \AddQuickBooksExternalIds\
• Inject sync into \InvoiceService\ (UpsertInvoice, UpdateInvoice, MarkPaid) and \OrganizationClientService\ (UpsertClient) as optional dependency — fire-and-forget safe via internal try/catch
• Supports sandbox and production QB environments via \IQuickBooksSettings.UseSandbox\

Behaviour

• No-ops silently if org has no QB connection
• QB failures are logged but never fail the JobFlow operation

Testing

• Build: 0 warnings, 0 errors
• Unit tests: 54/54 passed
• Migration applied locally