insights: re-use matching series for a query if they exist for scoped insights

Created by: leonore

currently for global series if we find matching insight series by query in the db for a new insight, we will just return those, saving computing time.

after we migrating all just-in-time insights to be backfilled this could apply to all insights, scoped or not: https://sourcegraph.com/github.com/sourcegraph/sourcegraph@b9a6cee518fdac7dc48545be2c050515aa4224a7/-/blob/enterprise/internal/insights/resolvers/insight_view_resolvers.go?L1027-1039=