refactor: prevent whole table scan while scheduling next date

- helps in concurrency isolation

(cherry picked from commit 21f4603144)

erpnext/accounts/doctype/process_period_closing_voucher_detail/process_period_closing_voucher_detail.py

@@ -1,7 +1,7 @@
 # Copyright (c) 2025, Frappe Technologies Pvt. Ltd. and contributors
 # For license information, please see license.txt
 
-# import frappe
+import frappe
 from frappe.model.document import Document
 
 
@@ -24,3 +24,10 @@ class ProcessPeriodClosingVoucherDetail(Document):
 	# end: auto-generated types
 
 	pass
+
+
+def on_doctype_update():
+	frappe.db.add_index(
+		"Process Period Closing Voucher Detail",
+		["parent", "status", "parentfield", "idx", "processing_date"],
+	)