Chore: upgrade prisma

[mipyykko]

Finally give in and upgrade Prisma to a modern version, hopping right from 2.x to 4.x. This called for replacing the deprecated nexus-plugin-prisma with a forked version, but the changes there have been relatively minor.

Main Prisma-related refactors in this PR are mainly just to rename the primary/foreign/unique keys and indexes to use the new default convention. Prisma schema is rewritten using the introspection feature, although some manual changes are still needed -- now the Postgres Decimal columns would be automatically cast to Decimal.js objects, which would just break things, so I force them to be Float as before.

Now that Prisma can be upgraded, we should also use its new features -- for example transactions are useful. Also, previously Prisma did not cascade delete even if the DB schema was set up to do so, but now it does.

[codecov]

Codecov Report

Merging #1190 (a1eadc8) into master (a9a548f) will decrease coverage by 0.27%.
The diff coverage is 34.86%.

@@            Coverage Diff             @@
##           master    #1190      +/-   ##
==========================================
- Coverage   65.51%   65.24%   -0.27%     
==========================================
  Files         124      124              
  Lines        5222     5280      +58     
  Branches     1160     1225      +65     
==========================================
+ Hits         3421     3445      +24     
- Misses       1677     1711      +34     
  Partials      124      124              

Flag	Coverage Δ
backend	65.24% <34.86%> (-0.27%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
backend/api/routes/completions.ts	58.02% <0.00%> (-0.37%)	⬇️
backend/api/routes/progress.ts	30.85% <0.00%> (-0.91%)	⬇️
...nsumer/common/userCourseProgress/BAI/completion.ts	14.28% <0.00%> (-0.43%)	⬇️
backend/graphql/Completion/model.ts	40.24% <0.00%> (-0.50%)	⬇️
backend/graphql/Completion/queries.ts	24.52% <0.00%> (-1.48%)	⬇️
backend/graphql/StudyModule/model.ts	55.55% <0.00%> (-2.14%)	⬇️
backend/graphql/Tag.ts	41.46% <0.00%> (-0.34%)	⬇️
backend/graphql/User/model.ts	35.75% <0.00%> (-0.89%)	⬇️
backend/graphql/UserCourseProgress.ts	30.25% <0.00%> (-0.26%)	⬇️
backend/graphql/UserCourseServiceProgress.ts	49.05% <0.00%> (ø)
... and 13 more

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