Set queue.yml thread back to 3

because I can see we're backing up on Event::WebhookDispatchJob,
PushNotificationJob, and Turbo::Streams jobs, all of which are
doing networking.

config/queue.yml

@@ -4,7 +4,7 @@ default: &default
       batch_size: 500
   workers:
     - queues: [ "default", "solid_queue_recurring", "backend", "webhooks" ]
-      threads: 1
+      threads: 3
       processes: <%= ENV.fetch("JOB_CONCURRENCY", Concurrent.physical_processor_count) %>
       polling_interval: 0.1