perf: use unbuffered_cursor for stock_ageing report (#39399)

perf: use unbuffered_cursor for stock stock_ageing report

Unbuffered cursor lets us load one row at a time in memory from mysql instead of lowing all at once.

This makes it possible to run the report for large number of SLEs.

erpnext/stock/report/stock_ageing/stock_ageing.py

@@ -233,6 +233,8 @@ class FIFOSlots:
 		"""
 
 		stock_ledger_entries = self.sle
+
+		with frappe.db.unbuffered_cursor():
 			if stock_ledger_entries is None:
 				stock_ledger_entries = self.__get_stock_ledger_entries()