Man, halfway reading this I was pissed off because I suspected something and then they confirmed it.
They wanted you to build them an app for free.
That's what this was.
Creating a Django app with Vue and Docker, testing, OAuth, setting up PostgreSQL, a few views, design, etc that's not a toy project nor a test for a job. That's a full solution for something.
For something they are selling, I would say.
The fact that the requirements were high, plus the small frame of time they gave you so they can kick you out because you "took too much time", plus what they asked for, and especially the fact that despite not liking (supposedly) what you did yet they require you to fix the errors...
They wanted somebody to build something for them, for free. Dead giveaway.
Django + Vue is my stack too. I'm way less experienced than you (1.5 years of Django, 9 months of Vue), but I work with people that have +15 years of experience and have been working with Django since the beta came out, so I recognize good code (despite not being able to write it myself 😃), and yours is top notch.
Don't pay attention to what they said, they just were sneaky. Don't think for a second you're "mediocre", you should be proud of what you did.
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