Consider adding pgvector extension

[pgvector](https://github.com/pgvector/pgvector) enables vector similarity search which is useful for AI applications. Support exists in Azure, AWS, and Supabase for this extension, indicating it is growing in popularity.

I had originally hoped to use Upstash Redis on Fly.io for a similarity search, but Upstash Redis does not support this functionality yet. (Redis has the RediSearch extension for vector similarity search).

Pgvector support provides another way to achieve this.