Prisma migrate

[berekuk]

This PR is for managing DB schema with Prisma Migrate, as planned in #33.

Things to note:

• npm run cli migrate is no more (I don't see much benefit in wrapping prisma migrate * in it)
• I removed the read-only database access entirely; there's just one pool now instead of readWritePool. I think RO access was broken anyway since we changed the databases, and I think there were zero active users for it (right?); it can be restored if necessary, but there must be better ways to achieve this
  • one is our future GraphQL API
  • another possibility is to implement a custom job which dumps the entire database state in an archive and exposes it through HTTP or stores on S3
• DB schema is kinda different from before
  • Prisma complained about some stuff, e.g. missing primary keys, so I had to change this; one example is that history table got a pk int autoincrement field
  • I also took the opportunity to change all fields to non-nulls (nulls in sql are confusing) and all json fields to jsonb
• new DB structure will require one final manual migration (implemented in src/backend/migrate/prepareForPrisma.ts)
• and also one prisma-based manual migration according to prisma's baselining docs
  • (I'll do both of these by hand when I'll merge this PR)

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[NunoSempere]

Looks good!