Add unique constraint on column `url` on table `CurrentVersions` in Postgres pipeline

[anteverse]

Hello, we've noticed performances issues when running lots of urls with the Postgres pipeline.

We would like to add a unique constraint on url column since:

• the select query filtered on the url expects 0 or 1 row
• the unique constraint will create a unique index that helps a lot performance wise

[anteverse]

@fhamborg Any opinion on that? I feel like we should never have duplicate urls in CurrentVersions table

[fhamborg]

thanks for the pr!

[anteverse]

Thanks for the quick merge!