🐛 Enforce case-insensitive username uniqueness

[aaronleopold]

Fixes #556

I've adjusted the username column manually to add COLLATE NOCASE according to https://www.prisma.io/docs/orm/prisma-client/queries/case-sensitivity#sqlite-provider. I've also adjusted the frontend validation to ignore case when comparing.

I have tested manual DB insertion/updates and confirmed the migration enforces case-insensitive usernames (e.g., oromei or OrOmEI are equal). I'll try to do another validation pass tomorrow before merging, though. Edit to add that I've also tested the migration doesn't blow up my user or existing preferences, which is good I guess lol

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

see 8 files with indirect coverage changes