feat(dashboard): add real-time in-app stats with Chart.js

- dashboard_stats_bp.py: endpoints /pos/api/dashboard/stats and
  /pos/api/dashboard/stats/employees (sales today/month, hourly,
  top products, employee productivity)
- dashboard-stats.js: renders hourly sales bar chart and top products
  doughnut chart using Chart.js
- chart.umd.min.js: vendored Chart.js v4.4.2

pos/blueprints/dashboard_stats_bp.py

@@ -0,0 +1,107 @@
+"""Dashboard Stats Blueprint — In-app real-time analytics.
+
+Endpoints for sales, productivity, and top products charts.
+"""
+from flask import Blueprint, request, jsonify, g
+from functools import wraps
+from datetime import datetime, timedelta
+from decimal import Decimal
+import json
+
+dashboard_stats_bp = Blueprint('dashboard_stats', __name__, url_prefix='/pos/api/dashboard')
+
+
+from middleware import require_auth
+
+
+class DecimalEncoder(json.JSONEncoder):
+    def default(self, o):
+        if isinstance(o, Decimal):
+            return float(o)
+        return super().default(o)
+
+
+@dashboard_stats_bp.route('/stats', methods=['GET'])
+@require_auth()
+def get_stats():
+    """Summary stats for today and this month."""
+    from tenant_db import get_tenant_db
+    db = get_tenant_db()
+    today = datetime.utcnow().date()
+    month_start = today.replace(day=1)
+
+    # Sales today
+    today_sales = db.execute(
+        """SELECT COUNT(*) as count, COALESCE(SUM(total), 0) as total
+           FROM sales WHERE DATE(created_at) = %s""", (today,)
+    ).fetchone()
+
+    # Sales this month
+    month_sales = db.execute(
+        """SELECT COUNT(*) as count, COALESCE(SUM(total), 0) as total
+           FROM sales WHERE DATE(created_at) >= %s""", (month_start,)
+    ).fetchone()
+
+    # Top 5 products today
+    top_products = db.execute(
+        """SELECT p.name, SUM(si.quantity) as qty, SUM(si.total) as revenue
+           FROM sale_items si
+           JOIN sales s ON si.sale_id = s.id_sale
+           JOIN parts p ON si.part_id = p.id_part
+           WHERE DATE(s.created_at) = %s
+           GROUP BY p.name
+           ORDER BY revenue DESC
+           LIMIT 5""", (today,)
+    ).fetchall()
+
+    # Hourly sales today (0-23)
+    hourly = db.execute(
+        """SELECT EXTRACT(HOUR FROM created_at)::int as hour,
+                  COUNT(*) as count, COALESCE(SUM(total), 0) as total
+           FROM sales WHERE DATE(created_at) = %s
+           GROUP BY hour ORDER BY hour""", (today,)
+    ).fetchall()
+    hourly_map = {row['hour']: {'count': row['count'], 'total': row['total']} for row in hourly}
+
+    return jsonify({
+        'today': {
+            'sales_count': today_sales['count'],
+            'sales_total': today_sales['total'],
+        },
+        'month': {
+            'sales_count': month_sales['count'],
+            'sales_total': month_sales['total'],
+        },
+        'top_products': [
+            {'name': row['name'], 'quantity': row['qty'], 'revenue': row['revenue']}
+            for row in top_products
+        ],
+        'hourly_sales': [
+            {'hour': h, 'count': hourly_map.get(h, {}).get('count', 0),
+             'total': hourly_map.get(h, {}).get('total', 0)}
+            for h in range(24)
+        ],
+    }, cls=DecimalEncoder)
+
+
+@dashboard_stats_bp.route('/stats/employees', methods=['GET'])
+@require_auth()
+def get_employee_stats():
+    """Sales per employee today."""
+    from tenant_db import get_tenant_db
+    db = get_tenant_db()
+    today = datetime.utcnow().date()
+    rows = db.execute(
+        """SELECT e.name, COUNT(s.id_sale) as sales, COALESCE(SUM(s.total), 0) as total
+           FROM sales s
+           JOIN employees e ON s.employee_id = e.id_employee
+           WHERE DATE(s.created_at) = %s
+           GROUP BY e.name
+           ORDER BY total DESC""", (today,)
+    ).fetchall()
+    return jsonify({
+        'employees': [
+            {'name': row['name'], 'sales': row['sales'], 'total': row['total']}
+            for row in rows
+        ]
+    }, cls=DecimalEncoder)