Code P0171?

The car listed above is an SV Pure drive with CVT transmission.
The car has had decent Maintenace since I have owned it since 2014. I do regular oil changes brake inspection and have most documentation of work that has been done on the vehicle since I have purchased from a local dealer.
I’ve done regular brake pad changes
(Front Rotors I was told I will need by end of august as well as new pads for the front. Currently about halfway down)
I’ve not had a tune up since 2017.
New CVT transmission 12/20.
Front left axle and front right axle replaced Jan and Feb 2022
Rear drums were done in 2018. At inspection a few days ago was told they aren’t even halfway down so I still have awhile left.
About 2 weeks ago the car would stop and idle at a light and all brake pressure would be lost. Pedal went to floor, and I would roll a hit. Then I would have to pump it two maybe three times and it would come back to normal.
When this began to happen at nearly every stop, I contacted my mechanic.
I was cautious to research and order both master cylinder as well as brake booster in the event both needed replacing.
Mechanic replaced booster first. Then bled all the brakes. The problem was still there
Then he replaced the master cylinder.
Problem of losing brake pressure at idle or
Stop went away.
However, when mechanic test drove it. It was okay, but then I test drove it and the check engine light came
On. I want to know if that was due to him having to take the battery out for about 35 mins when he was replacing the parts.
Anyway, my husband immediately got his OBD reader, and it read
P0171 lean bank 1 or something to that effect.
He cleared the code. That was 4 days ago, and it hasn’t come back.
However - when I drove the car the next day at highway speeds when I go over 50 MPH, the car sometimes whistles high pitched but it’s not loud. It happens mostly when I accelerate. When I brake the sound goes away. And it
doesn’t happen All the time either
I noticed the pedal does seem to not have as much hardness if that makes sense. It’s like going down closer to the floor when I stop. Can this be because of the front pads? I barely drove though over last 4 days it’s not like I went on a 1,000-mile trip or something.
What is this p0171 code and what does the high pitch intermittent whistle at high speeds mean?
I told the mechanic the day after he did the repairs and he asked me to bring it by to see if he could hear it, but I wasn’t able to make it out his way.
Does Anyone know what this is
Is it dangerous to drive the car this way?
Everything else seems to be ok with the way it’s running currently.
Thank you for any insight that you could provide!