feat: MercadoLibre integration + inventory bulk publish + WhatsApp bridge fixes

- Add MercadoLibre OAuth, listings, orders, webhooks and category search
- New marketplace_external_bp.py, meli_service.py, marketplace_external_service.py
- New marketplace_external.html/js with ML management UI
- Inventory: bulk publish to ML with category autocomplete, listing type and shipping selectors
- Inventory: new .btn--meli styles, select/label CSS fixes
- WhatsApp bridge: rate limiting, 440/515/408 error handling, stale watchdog
- DB migration v3.4_meli_integration.sql for marketplace_listings, orders, sync_queue
- Add Celery tasks for ML sync and webhook processing
- Sidebar: MercadoLibre navigation link

pos/blueprints/invoicing_bp.py

@@ -397,3 +397,30 @@ def get_sale_pdf(sale_id):
         'customer': customer,
         'cfdi': cfdi_info,
     })
+
+
+@invoicing_bp.route('/stats', methods=['GET'])
+@require_auth('invoicing.read')
+def api_invoicing_stats():
+    """Return counts for tab badges: invoices, credit notes, payment complements, cancellations."""
+    conn = get_tenant_conn(g.tenant_id)
+    cur = conn.cursor()
+
+    cur.execute("""
+        SELECT
+            COUNT(*) FILTER (WHERE type = 'ingreso' AND status IN ('pending', 'stamped', 'retry')) as facturas,
+            COUNT(*) FILTER (WHERE type = 'egreso' AND status IN ('pending', 'stamped', 'retry')) as notas_credito,
+            COUNT(*) FILTER (WHERE type = 'pago' AND status IN ('pending', 'stamped', 'retry')) as complementos,
+            COUNT(*) FILTER (WHERE status = 'cancelled') as cancelaciones
+        FROM cfdi_queue
+    """)
+    row = cur.fetchone()
+    cur.close()
+    conn.close()
+
+    return jsonify({
+        'facturas': row[0] or 0,
+        'notas_credito': row[1] or 0,
+        'complementos': row[2] or 0,
+        'cancelaciones': row[3] or 0,
+    })