Performance: pluck().uniq instead of distinct.pluck in VisitsInLastDayJob

## Description

In `app/jobs/visits_in_last_day_job.rb:8`:

```ruby
account_ids = Ahoy::Event.where("time > ?", 1.day.ago).pluck(:user_id).uniq
```

This loads all event user_ids into Ruby memory before deduplicating.

## Recommendation

Use database-level deduplication:

```ruby
account_ids = Ahoy::Event.where("time > ?", 1.day.ago).distinct.pluck(:user_id)
```

## Severity

Medium