Add extra-top-period for Reddit

### Description

Right now, Reddit lets you filter top by hour, day, week, month, year, all time.

If we want to filter by top in last 3 days, we can do something like:
1. fetch top in past week
2. filter out posts older than 3 days.

Currently, I'm using the `week` filter and just manually scanning through the list to see what's been updated within a desired time period.

Start by locating the Reddit integration and its handling of top-period filters. Check how Reddit's weekly results expose post timestamps, then verify that filtering to three days excludes older posts while preserving the existing periods. Done means a configurable three-day period works reliably and is covered by the relevant tests.

Written by the indexing model from the issue text.