[FEAT]: Implement (Task-) Garbage Collector

[matteokosina]

Expected Result:

• separate job (maybe via special go-routing?)
• run on a regular basis (-> evaluate a suitable schedule -> every min, 2min, 10sec, ...)
• logic should something similar to this pseudo-code:

for every task in db:{
   if task.status == successful || task.status == failed{
       if (task.created + task.retention) < Date.now{ // if retention is in past
           remove(task)
       }
   }
}