Ruby: "do this task eventually"

Answer 1

I think you'd generally do this kind of stuff with a cronjob or your system's equivalent functionality. Have a script that goes through all the links in your database and check whether they're valid. Maybe have an hourly one that does it for new links and have the others set to daily or weekly so you're not doing too much work.

Answer 2

Delayed Job is great for queuing tasks in Rails apps.