Port findings platform to use postgresql

We want the findings platform to use postgresql to store ooni findings/incidents, since our clickhouse database is not backed up. This is part of https://github.com/ooni/backend/issues/814. We also want to replace the current API routes we have from `api/v1/incidents` to `api/v1/findings` as suggested here: https://github.com/ooni/backend/pull/850#discussion_r1634922142