Cache boosts with their message instead of one by one

Every boost had its own fragment, nested inside the message fragment that
already holds it. With a cold cache, a read and a write per boost plus a
read_multi and a write_multi per message: a room page of 40 messages with
120 boosts takes ~410 ms instead of ~290 ms. Redis runs without persistence
(config/redis.conf), so every page is cold after a restart.

A boost touches its message, so adding or removing one rewrites the message
fragment anyway. The inner fragments only paid off when a message with many
boosts was drawn again, and there they hid a lookup per booster. The boosters
are now loaded with the boosts, one query per message.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0142qgjggdJ2KDdGk7RF9Xm9

test/controllers/messages_caching_test.rb

@@ -27,6 +27,32 @@ class MessagesCachingTest < ActionDispatch::IntegrationTest
     end
   end
 
+  test "boosts are cached inside their message instead of one fragment each" do
+    with_memory_cache do
+      cache_keys = []
+      subscriber = ActiveSupport::Notifications.subscribe(/\Acache_(read|read_multi|write|write_multi)\.active_support\z/) do |*, payload|
+        cache_keys.concat(payload[:key].is_a?(Hash) ? payload[:key].keys : Array(payload[:key]))
+      end
+
+      get room_messages_url(rooms(:watercooler))
+      assert_response :success
+      assert_select "#" + dom_id(boosts(:fourth_by_bender))
+      assert_select "#" + dom_id(boosts(:thirteenth))
+      assert_empty cache_keys.grep(%r{messages/boosts/_boost})
+
+      boost = messages(:fourth).boosts.create! booster: users(:jason), content: "🎉"
+      get room_messages_url(rooms(:watercooler))
+      refreshed = response.body
+
+      Rails.cache.clear
+      get room_messages_url(rooms(:watercooler))
+      assert_equal refreshed, response.body
+      assert_select "##{dom_id(messages(:fourth))} ##{dom_id(boost)}", text: /🎉/
+    ensure
+      ActiveSupport::Notifications.unsubscribe(subscriber)
+    end
+  end
+
   private
     def with_memory_cache
       old_cache = Rails.cache