Use to postgres built-in fulltext search

[tobiasdiez]

Currently, we get all documents matching the search expression and then filtering them by hand. To increase performance, one should instead use postgres fulltext search. Prisma is now supporting this: https://www.prisma.io/docs/concepts/components/prisma-client/full-text-search
But there are still a few limitations (e.g. the index needs to be handled externally). Thus, let's wait a bit until that feature is fully implemented.
Should also sort by relevance https://prisma.io/docs/concepts/components/prisma-client/filtering-and-sorting#sort-by-relevance-postgresql

Depends on:

• Allow adding FULLTEXT index in the schema prisma/prisma#3325
• Support @unique on nullable fields for databases that support it prisma/prisma#3387
• Support fulltext search using indexes in PostgreSQL prisma/prisma#10386