How to insert one row before each day, with value based on last row of previous day?

Question

I am struggling with one thing in Pandas. I am not sure how to add a new rows at the beginning of each day, without using for .. in, what takes a lot of time.

Below is a table, where date_time is a timestamp created with to_datetime. For each day, before 09:00:00+01:00 I need to add a row with 08:00:00+01:00 and copy last value from previous day.

I will be grateful for any help.

                      date_time  value
20437 2022-02-10 09:00:00+01:00  80.80 #<-before this
20438 2022-02-10 10:00:00+01:00  80.20
20439 2022-02-10 11:00:00+01:00  80.20
20440 2022-02-10 12:00:00+01:00  80.70
20441 2022-02-10 13:00:00+01:00  81.10
20442 2022-02-10 14:00:00+01:00  81.00
20443 2022-02-10 15:00:00+01:00  80.15
20444 2022-02-10 16:00:00+01:00  80.25
20445 2022-02-10 17:00:00+01:00  79.50
20446 2022-02-11 09:00:00+01:00  79.50 #<-before this
20447 2022-02-11 10:00:00+01:00  79.00
20448 2022-02-11 11:00:00+01:00  78.70
20449 2022-02-11 12:00:00+01:00  78.90
20450 2022-02-11 13:00:00+01:00  79.00
20451 2022-02-11 14:00:00+01:00  79.05
20452 2022-02-11 15:00:00+01:00  79.40
20453 2022-02-11 16:00:00+01:00  79.85
20454 2022-02-11 17:00:00+01:00  79.50