PERF: Add index_topics_on_category_id (#37022)

Joining the `topics` table against the `categories` table is a common
thing we do and the missing index on the `category_id` foreign key is
hurting performance on sites with lots of rows in the `topics` and
`categories` tables.

app/models/topic.rb

@@ -2304,6 +2304,7 @@ end
 #  idxtopicslug                            (slug) WHERE ((deleted_at IS NULL) AND (slug IS NOT NULL))
 #  index_topics_on_bannered_until          (bannered_until) WHERE (bannered_until IS NOT NULL)
 #  index_topics_on_bumped_at_public        (bumped_at) WHERE ((deleted_at IS NULL) AND ((archetype)::text <> 'private_message'::text))
+#  index_topics_on_category_id             (category_id) WHERE ((deleted_at IS NULL) AND ((archetype)::text <> 'private_message'::text))
 #  index_topics_on_created_at_and_visible  (created_at,visible) WHERE ((deleted_at IS NULL) AND ((archetype)::text <> 'private_message'::text))
 #  index_topics_on_external_id             (external_id) UNIQUE WHERE (external_id IS NOT NULL)
 #  index_topics_on_id_and_deleted_at       (id,deleted_at)

db/migrate/20260109041508_add_index_category_on_topics.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+class AddIndexCategoryOnTopics < ActiveRecord::Migration[8.0]
+  disable_ddl_transaction!
+
+  def up
+    execute "DROP INDEX IF EXISTS index_topics_on_category_id;"
+    execute "CREATE INDEX CONCURRENTLY index_topics_on_category_id ON topics (category_id) WHERE deleted_at IS NULL AND (archetype <> 'private_message');"
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end