chore: Fix connection pool (#6005)

We want to allocate as much database connection to the pool to match the sidekiq concurrency configuration.
ref: https://maxencemalbois.medium.com/the-ruby-on-rails-database-connections-pool-4ce1099a9e9f

fixes: #6004

config/database.yml

@@ -1,9 +1,10 @@
 default: &default
   adapter: postgresql
   encoding: unicode
-  pool: <%= ENV.fetch("RAILS_MAX_THREADS") { 5 } %>
   host: <%= ENV.fetch('POSTGRES_HOST', 'localhost') %>
   port: <%= ENV.fetch('POSTGRES_PORT', '5432') %>
+   # ref: https://github.com/mperham/sidekiq/issues/2985#issuecomment-531097962
+  pool: <%= Sidekiq.server? ? Sidekiq.options[:concurrency] : ENV.fetch('RAILS_MAX_THREADS', 5) %>
 
 development:
   <<: *default

config/sidekiq.yml

@@ -4,7 +4,7 @@
 # pick it up automatically.
 ---
 :verbose: false
-:concurrency: 5
+:concurrency: <%= ENV.fetch("SIDEKIQ_CONCURRENCY", 5) %>
 :timeout: 25
 :max_retries: 3
 
@@ -30,6 +30,6 @@
 
 # you can override concurrency based on environment
 production:
-  :concurrency: 10
+  :concurrency: <%= ENV.fetch("SIDEKIQ_CONCURRENCY", 10) %>
 staging:
-  :concurrency: 5
+  :concurrency: <%= ENV.fetch("SIDEKIQ_CONCURRENCY", 5) %>