OPCIÓN A: A2 Virtual Scroll + A3 Celery + A4 asyncpg PoC + A5 particionamiento

A2 — Virtual scroll en tablas grandes:
- Nuevo helper VirtualScroll en pos/static/js/virtual-scroll.js
- inventory.js: tabla de productos con virtual scroll
- customers.js: tabla de clientes con virtual scroll
- fleet.js: renderMaintenance() y renderHistory() con virtual scroll
- Templates envueltos en .vs-container para scroll

A3 — Celery worker queue:
- pos/celery_app.py + pos/tasks.py (warm cache, bulk import, reports)
- Blueprint tasks_bp.py con endpoints /pos/api/tasks/*
- Script scripts/start_celery.sh

A4 — asyncpg + Quart PoC:
- pos/async_catalog.py: endpoint /pos/api/catalog/async-search
- scripts/benchmark_async_catalog.py: benchmark Flask vs Quart

A5 — Particionar vehicle_parts:
- scripts/partition_vehicle_parts.py: migración segura por hash (16 particiones)
- Soporta --dry-run, --skip-swap, --skip-drop

Tests: 36/36 pasando

pos/static/js/customers.js

@@ -94,45 +94,53 @@ const Customers = (() => {
         }
     }
 
+    function renderCustomerRow(c) {
+        const tier = tierMap[c.price_tier] || 'Mostrador';
+        const tClass = tierClass[c.price_tier] || 'mostrador';
+        const limit = parseFloat(c.credit_limit || 0);
+        const balance = parseFloat(c.credit_balance || 0);
+        const available = Math.max(0, limit - balance);
+        const usedPct = limit > 0 ? Math.round((balance / limit) * 100) : 0;
+        const creditClass = usedPct >= 80 ? 'none' : usedPct >= 60 ? 'low' : '';
+        const num = String(c.id).padStart(5, '0');
+        const selClass = (currentCustomer && currentCustomer.id === c.id) ? 'selected' : '';
+        return '<tr class="' + selClass + '" onclick="selectCustomer(' + c.id + ')">' +
+            '<td class="cell-num">' + num + '</td>' +
+            '<td>' +
+                '<div class="cell-name">' + (c.name || '') + '</div>' +
+                '<div class="cell-name-sub hide-mobile">' + (c.email || '') + '</div>' +
+            '</td>' +
+            '<td class="cell-rfc hide-mobile">' + (c.rfc || '-') + '</td>' +
+            '<td class="hide-mobile">' + (c.phone || '-') + '</td>' +
+            '<td class="hide-mobile" style="font-size:var(--text-caption);color:var(--color-text-secondary);">' + (c.email || '-') + '</td>' +
+            '<td><span class="tipo-chip tipo-chip--' + tClass + '">' + tier + '</span></td>' +
+            '<td class="cell-credit ' + creditClass + '">' + fmt(available) + '</td>' +
+            '<td class="cell-date hide-mobile">' + formatDate(c.last_purchase || c.created_at) + '</td>' +
+            '<td>' + statusBadge(c) + '</td>' +
+            '</tr>';
+    }
+
+    var customersVS = null;
+
     function renderTable(customers) {
         const tbody = document.getElementById('customersBody');
         if (!tbody) return;
-        tbody.innerHTML = '';
 
         if (!customers || customers.length === 0) {
             tbody.innerHTML = '<tr><td colspan="9" style="text-align:center;padding:var(--space-8);color:var(--color-text-muted);">Sin resultados.</td></tr>';
             return;
         }
 
-        customers.forEach((c, idx) => {
-            const tier = tierMap[c.price_tier] || 'Mostrador';
-            const tClass = tierClass[c.price_tier] || 'mostrador';
-            const limit = parseFloat(c.credit_limit || 0);
-            const balance = parseFloat(c.credit_balance || 0);
-            const available = Math.max(0, limit - balance);
-            const usedPct = limit > 0 ? Math.round((balance / limit) * 100) : 0;
-            const creditClass = usedPct >= 80 ? 'none' : usedPct >= 60 ? 'low' : '';
-            const num = String(c.id).padStart(5, '0');
-
-            const tr = document.createElement('tr');
-            if (currentCustomer && currentCustomer.id === c.id) tr.className = 'selected';
-            tr.onclick = () => selectCustomer(c.id);
-            tr.innerHTML = `
-                <td class="cell-num">${num}</td>
-                <td>
-                    <div class="cell-name">${c.name || ''}</div>
-                    <div class="cell-name-sub hide-mobile">${c.email || ''}</div>
-                </td>
-                <td class="cell-rfc hide-mobile">${c.rfc || '-'}</td>
-                <td class="hide-mobile">${c.phone || '-'}</td>
-                <td class="hide-mobile" style="font-size:var(--text-caption);color:var(--color-text-secondary);">${c.email || '-'}</td>
-                <td><span class="tipo-chip tipo-chip--${tClass}">${tier}</span></td>
-                <td class="cell-credit ${creditClass}">${fmt(available)}</td>
-                <td class="cell-date hide-mobile">${formatDate(c.last_purchase || c.created_at)}</td>
-                <td>${statusBadge(c)}</td>
-            `;
-            tbody.appendChild(tr);
-        });
+        if (!customersVS) {
+            customersVS = new VirtualScroll({
+                container: tbody,
+                rowHeight: 52,
+                buffer: 3,
+                renderRow: renderCustomerRow,
+                emptyHtml: '<tr><td colspan="9" style="text-align:center;padding:var(--space-8);color:var(--color-text-muted);">Sin resultados.</td></tr>'
+            });
+        }
+        customersVS.setData(customers);
     }
 
     function renderPagination(pag) {