feat: 類似度算出バッチ修正，ほか（#228） (#232)

#228

#228

#228

Co-authored-by: miteruzo <miteruzo@naver.com>
Reviewed-on: #232

backend/db/migrate/20260121213900_add_index_to_post_similarities.rb

@@ -0,0 +1,9 @@
+class AddIndexToPostSimilarities < ActiveRecord::Migration[8.0]
+  def change
+    remove_index :post_similarities, name: 'index_post_similarities_on_post_id'
+
+    add_index :post_similarities, [:post_id, :cos],
+              order: { cos: :desc },
+              name: 'index_post_similarities_on_post_id_and_cos'
+  end
+end