[webapp] Use Prisma pgvector support

[hijohnnylin]

Describe the feature you're requesting. If it's related to a problem, please describe it.

We currently use a hacky workaround to for explanation embeddings in the db. But as of last month Prisma now supports pgvector in early access: https://www.prisma.io/blog/orm-6-13-0-ci-cd-workflows-and-pgvector-for-prisma-postgres