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/virtual-scroll.js

@@ -0,0 +1,155 @@
+/**
+ * virtual-scroll.js — Lightweight vanilla-JS virtual scroll helper.
+ * Supports <div> containers and <tbody> tables.
+ *
+ * Usage:
+ *   var vs = new VirtualScroll({
+ *     container: document.getElementById('myTableBody'),
+ *     rowHeight: 48,
+ *     buffer: 5,
+ *     renderRow: function(item, index) { return '<tr>...</tr>'; }
+ *   });
+ *   vs.setData(arrayOfItems);
+ */
+(function(window) {
+    'use strict';
+
+    function VirtualScroll(opts) {
+        this.container = opts.container;
+        this.rowHeight = opts.rowHeight || 48;
+        this.buffer = opts.buffer || 5;
+        this.renderRow = opts.renderRow || function() { return ''; };
+        this.emptyHtml = opts.emptyHtml || '';
+        this.data = [];
+        this._scrollHandler = this._onScroll.bind(this);
+        this._resizeHandler = this._onResize.bind(this);
+        this._isTbody = this.container.tagName === 'TBODY';
+        this._init();
+    }
+
+    VirtualScroll.prototype._init = function() {
+        var c = this.container;
+        if (!this._isTbody) {
+            c.style.overflowY = 'auto';
+            c.style.position = 'relative';
+            if (!c.style.maxHeight && !c.style.height) {
+                c.style.maxHeight = '60vh';
+            }
+        } else {
+            // For tbody, scroll is on the parent element (table wrapper)
+            var table = c.closest('table');
+            if (table) {
+                var wrapper = table.parentElement;
+                if (wrapper && wrapper.classList.contains('vs-container')) {
+                    wrapper.addEventListener('scroll', this._scrollHandler, { passive: true });
+                }
+            }
+        }
+        window.addEventListener('resize', this._resizeHandler, { passive: true });
+    };
+
+    VirtualScroll.prototype.setData = function(data) {
+        this.data = data || [];
+        this._render();
+    };
+
+    VirtualScroll.prototype.refresh = function() {
+        this._render();
+    };
+
+    VirtualScroll.prototype._onScroll = function() {
+        this._render();
+    };
+
+    VirtualScroll.prototype._onResize = function() {
+        this._render();
+    };
+
+    VirtualScroll.prototype._getScrollTop = function() {
+        if (this._isTbody) {
+            var table = this.container.closest('table');
+            if (table) {
+                var wrapper = table.parentElement;
+                if (wrapper && wrapper.classList.contains('vs-container')) {
+                    return wrapper.scrollTop;
+                }
+            }
+            return 0;
+        }
+        return this.container.scrollTop;
+    };
+
+    VirtualScroll.prototype._getContainerHeight = function() {
+        if (this._isTbody) {
+            var table = this.container.closest('table');
+            if (table) {
+                var wrapper = table.parentElement;
+                if (wrapper && wrapper.classList.contains('vs-container')) {
+                    return wrapper.clientHeight;
+                }
+            }
+            return 600;
+        }
+        return this.container.clientHeight;
+    };
+
+    VirtualScroll.prototype._render = function() {
+        var data = this.data;
+        var rowH = this.rowHeight;
+        var buffer = this.buffer;
+
+        if (!data.length) {
+            if (this._isTbody) {
+                this.container.innerHTML = this.emptyHtml;
+            } else {
+                this.container.innerHTML = this.emptyHtml;
+            }
+            return;
+        }
+
+        var scrollTop = this._getScrollTop();
+        var containerHeight = this._getContainerHeight();
+        var startIdx = Math.max(0, Math.floor(scrollTop / rowH) - buffer);
+        var endIdx = Math.min(data.length, Math.ceil((scrollTop + containerHeight) / rowH) + buffer);
+
+        var html = '';
+        if (this._isTbody) {
+            // Top spacer row
+            var topSpacerHeight = startIdx * rowH;
+            if (topSpacerHeight > 0) {
+                html += '<tr style="height:' + topSpacerHeight + 'px;"><td colspan="99" style="padding:0;border:0;"></td></tr>';
+            }
+            for (var i = startIdx; i < endIdx; i++) {
+                html += this.renderRow(data[i], i);
+            }
+            // Bottom spacer row
+            var bottomSpacerHeight = (data.length - endIdx) * rowH;
+            if (bottomSpacerHeight > 0) {
+                html += '<tr style="height:' + bottomSpacerHeight + 'px;"><td colspan="99" style="padding:0;border:0;"></td></tr>';
+            }
+        } else {
+            for (var j = startIdx; j < endIdx; j++) {
+                html += this.renderRow(data[j], j);
+            }
+        }
+
+        this.container.innerHTML = html;
+    };
+
+    VirtualScroll.prototype.destroy = function() {
+        if (this._isTbody) {
+            var table = this.container.closest('table');
+            if (table) {
+                var wrapper = table.parentElement;
+                if (wrapper && wrapper.classList.contains('vs-container')) {
+                    wrapper.removeEventListener('scroll', this._scrollHandler);
+                }
+            }
+        } else {
+            this.container.removeEventListener('scroll', this._scrollHandler);
+        }
+        window.removeEventListener('resize', this._resizeHandler);
+    };
+
+    window.VirtualScroll = VirtualScroll;
+})(window);