Finish Prisma support

[akd-io]

Follow-up from #243 / #258. The Prisma plugin was merged as WIP. Remaining work:

• Bump prisma and @prisma/client to latest (currently pinned to ^4.16.0, now at v6.x)
• Add DATABASE_URL environment variable instead of hardcoding url in schema.prisma
• Add todos about switching database provider (per Add support for Prisma #243)
• Add npm scripts (prisma:generate, prisma:migrate, prisma:seed)
• Add --prisma to E2E "all-flags" tests
• Add dedicated Prisma E2E test