feat(atlas): import catalog, customers and historical sales + viewer

- Add scripts/import_atlas_data.py to load Atlas data from Excel files
- Import 6,206 inventory items, 251 customers and 4,582 historical sales
- Create historical_sales table in tenant DB
- Add /pos/historical-sales page and /pos/api/historical-sales endpoint
- Link in reports sidebar for easy access

pos/blueprints/pos_bp.py

@@ -232,6 +232,83 @@ def list_sales():
     })
 
 
+@pos_bp.route('/historical-sales', methods=['GET'])
+@require_auth('pos.view')
+def list_historical_sales():
+    """List imported historical sales (read-only reference).
+
+    Query params:
+        date_from: YYYY-MM-DD
+        date_to: YYYY-MM-DD
+        customer: partial customer name
+        page: int (default 1)
+        per_page: int (default 50, max 200)
+    """
+    conn = get_tenant_conn(g.tenant_id)
+    cur = conn.cursor()
+
+    page = int(request.args.get('page', 1))
+    per_page = min(int(request.args.get('per_page', 50)), 200)
+
+    where_clauses = ["1=1"]
+    params = []
+
+    date_from = request.args.get('date_from')
+    date_to = request.args.get('date_to')
+    customer = request.args.get('customer')
+
+    if date_from:
+        where_clauses.append("sale_date >= %s")
+        params.append(date_from)
+    if date_to:
+        where_clauses.append("sale_date <= %s")
+        params.append(date_to)
+    if customer:
+        where_clauses.append("customer_name ILIKE %s")
+        params.append(f"%{customer}%")
+
+    where = " AND ".join(where_clauses)
+
+    cur.execute(f"SELECT count(*) FROM historical_sales WHERE {where}", params)
+    total = cur.fetchone()[0]
+
+    cur.execute(f"""
+        SELECT id, external_document_id, document_no, sale_date, customer_name,
+               total, subtotal, amount_paid, payment_method, discount, balance,
+               raw_payment_code
+        FROM historical_sales
+        WHERE {where}
+        ORDER BY sale_date DESC, id DESC
+        LIMIT %s OFFSET %s
+    """, params + [per_page, (page - 1) * per_page])
+
+    rows = []
+    for r in cur.fetchall():
+        rows.append({
+            'id': r[0],
+            'external_document_id': r[1],
+            'document_no': r[2],
+            'sale_date': str(r[3]) if r[3] else None,
+            'customer_name': r[4],
+            'total': float(r[5]) if r[5] else 0,
+            'subtotal': float(r[6]) if r[6] else 0,
+            'amount_paid': float(r[7]) if r[7] else 0,
+            'payment_method': r[8],
+            'discount': float(r[9]) if r[9] else 0,
+            'balance': float(r[10]) if r[10] else 0,
+            'raw_payment_code': r[11],
+        })
+
+    cur.close()
+    conn.close()
+
+    total_pages = (total + per_page - 1) // per_page
+    return jsonify({
+        'data': rows,
+        'pagination': {'page': page, 'per_page': per_page, 'total': total, 'total_pages': total_pages}
+    })
+
+
 @pos_bp.route('/sales/<int:sale_id>', methods=['GET'])
 @require_auth('pos.view')
 def get_sale(sale_id):