hotfix: corrige 500 en /customers y /historical-sales

- customers_bp: last_purchase no existe como columna; se calcula via subquery a sales.
- pos_bp: historical_sales no existe en tenants sin importacion; endpoint devuelve lista vacia en lugar de 500.
- Servicio reiniciado y validado.

Tests: 35 passed

pos/blueprints/pos_bp.py

@@ -338,6 +338,16 @@ def list_historical_sales():
 
     where = " AND ".join(where_clauses)
 
+    # Defensive: the historical_sales table is created on demand by imports.
+    # If it does not exist yet, return an empty list instead of a 500.
+    cur.execute("SELECT to_regclass('historical_sales')")
+    if cur.fetchone()[0] is None:
+        cur.close(); conn.close()
+        return jsonify({
+            'data': [],
+            'pagination': {'page': page, 'per_page': per_page, 'total': 0, 'total_pages': 0}
+        })
+
     cur.execute(f"SELECT count(*) FROM historical_sales WHERE {where}", params)
     total = cur.fetchone()[0]