Dev & Test internal improvements

[blocknotes]

In this PR:

• add query comments (using annotate);
• setup Bullet gem for dev and test;
• minor improvements to the dummy app.

---

ℹ️ bullet is disabled only in specific spec due to an offense that requires some investigation:

     # spec/integration/attachments_spec.rb:15
     Bullet::Notification::UnoptimizedQueryError:
       user: app
        
       AVOID eager loading detected
         ActiveStorage::Attachment => [:record]
         Remove from your query: .includes([:record])