Fix backend business logic and remove hardcoded IDs

- Add customerId to SaleRequest DTO
- Update SaleService to use AuthenticationHelper for employee attribution
- Populate sale.customer when customerId provided in request
- Fix AdoptionController to use authenticated customer instead of hardcoded ID 1
- Fix AppointmentController to use authenticated customer instead of hardcoded ID 1
- Fix RefundController to use authenticated customer instead of hardcoded ID 1
- Update data.sql sales to include customer linkage for refund testing
- Update Postman collection sale creation with customerId and items

petshop-api.postman_collection.json

@@ -509,7 +509,7 @@
             ],
             "body": {
               "mode": "raw",
-              "raw": "{\n  \"employeeId\": 1,\n  \"storeId\": 1,\n  \"totalAmount\": 100.0,\n  \"paymentMethod\": \"Card\",\n  \"isRefund\": false\n}"
+              "raw": "{\n  \"storeId\": 1,\n  \"paymentMethod\": \"Card\",\n  \"customerId\": 1,\n  \"items\": [\n    {\n      \"prodId\": 1,\n      \"quantity\": 2\n    },\n    {\n      \"prodId\": 2,\n      \"quantity\": 1\n    }\n  ],\n  \"isRefund\": false\n}"
             }
           }
         }