fix(inbound/buy): 批号自增改为按物料精准取最近一条

- 新增 get_latest_batch_record_by_base_id：SQL 直接 filter(base_id)+order by in_date/id desc first()，O(1) 不分页
- 新增 GET /inbound/buy/latest-record
- 修复原“拉全表前1000条再前端过滤”导致的历史被截断→批号退回000001/重复入库被拦（如 CCAB0029）

inventory-backend/app/api/v1/inbound/buy.py

@@ -353,3 +353,18 @@ def get_last_location():
 
     location = BuyInboundService.get_last_location_by_base_id(base_id)
     return jsonify({"code": 200, "msg": "success", "data": {"location": location}})
+
+
+@inbound_buy_bp.route('/latest-record', methods=['GET'])
+@permission_required('inbound_buy')
+def get_latest_batch_record():
+    """
+    按物料精准取最近一次采购入库记录（批号自增用）。
+    返回 data: {'has_history': bool, 'mode': 'serial'|'batch', 'latest_batch': str}
+    """
+    base_id = request.args.get('base_id', type=int)
+    if not base_id:
+        return jsonify({"code": 400, "msg": "base_id required"}), 400
+
+    data = BuyInboundService.get_latest_batch_record_by_base_id(base_id)
+    return jsonify({"code": 200, "msg": "success", "data": data})