feat(pos): migrate CFDI timbrado from Horux to Facturapi
- Add Facturapi REST service (invoices, customers, orgs, cancel, downloads)
- Add JSON payload builder for ingreso/egreso/pago/global invoices
- Replace XML queue with Facturapi JSON queue (payload_unsigned, external_id)
- Update invoicing blueprint with Facturapi config and download endpoints
- Update global invoice service to use Facturapi payloads
- Add migration v4.3_facturapi.sql and tenant rollout script
- Update invoicing UI: payload preview, PDF/XML downloads, PAC status panel
- Add FACTURAPI_USER_KEY to .env.example