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(Please see edit below; problem might actually be with brake lever)

I'd like to know if a temporary resolution is possible for my stuck rear brake cable.

Picture:
https://drive.google.com/file/d/1JceR28W6Pot9i5v5Y_v3WtEV49UXqhvD/view?usp=sharing

Video:
https://drive.google.com/file/d/1EmGB78BphkK4k27j4BNouMpIK31A31ST/view?usp=sharing

I'm taking it into the shop but the soonest they can take my bike is Tuesday. I have no experience in bicycle repair. The bike is only half a year old, but I've been riding it in winter. I clean the brakes (excluding the pads) every one or two weeks with a soft bottle brush and diluted dish soap. I have never lubricated any brake parts yet, though.

Edit: I might have been wrong on the location of the malfunction. The brake lever seemed stuck, and I assumed that was because the brake was stuck, but after squeezing it very hard it seems to work to some extent. It makes a creaking noise and doesn't move as freely or with as much range as the front brake.

From the video, it looks like the cable has frayed inside the outer housing or something like that. I doubt there's anything you can do except get the bike shop to replace the cable.

Although most of the braking comes from the front brake, I woudln't recommend riding the bike in this state: you'd have no back-up if anything went wrong with the front brake.

Looks like excessive friction between cable and cable housing. Probably caused by a frayed cable or a kink in the housing. Is the cable housing damaged somewhere?

I’d try unclamping the cable at the brake and then check if the brake moves freely. You can then also unhook the cable at the brake lever and check if it moves freely as well. This would prove that the cable (or its housing) is the culprit. Light, penetrating oil (e.g. WD-40) could temporarily help to get it moving again, but if it’s damaged it could fail completely at any moment.

You could replace the cable yourself, it’s quite easy. Replacing the housing is a bit harder and works best with special tools but if you are only replacing it you already know where it should go and how long it has to be.

Riding without a rear brake is relatively safe (as long as the front brake works properly). Just make sure you remember that you don’t have a working rear brake, especially when indicating a left turn (assuming your rear brake is on your right hand).

In addition, that cable looks dirty to me in the picture and video. Maybe it's just the lighting? But if you have anything like WD-40, you can spray it on the cable and see if it loosens up. It's worth trying as it won't hurt anything.

If water gets into the cable housing when you're cleaning the bike, something like this could happen as the insides of the cable might rust up.