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

pos/templates/invoicing.html

@@ -865,6 +865,20 @@
               </div>
             </div>
 
+            <!-- FACTURAPI STATUS -->
+            <div class="config-section" style="grid-column: span 2;">
+              <div class="config-section__header">
+                <svg viewBox="0 0 24 24">
+                  <rect x="3" y="11" width="18" height="11" rx="2" ry="2"/>
+                  <path d="M7 11V7a5 5 0 0 1 10 0v4"/>
+                </svg>
+                <span class="config-section__title">Facturapi (PAC)</span>
+              </div>
+              <div class="config-section__body" id="facturapi-status">
+                <p style="color:var(--color-text-muted);">Cargando estado de Facturapi...</p>
+              </div>
+            </div>
+
             <!-- CONFIGURACIÓN DE SERIES — full width -->
             <div class="config-section" style="grid-column: span 2;">
               <div class="config-section__header">