feat: 上位タグ（#64） (#173)

#64 おそらく完成

Merge remote-tracking branch 'origin/main' into feature/064

#64 バックエンドぼちぼち

Co-authored-by: miteruzo <miteruzo@naver.com>
Reviewed-on: #173

backend/db/migrate/20251009222200_create_tag_implications.rb

@@ -0,0 +1,9 @@
+class CreateTagImplications < ActiveRecord::Migration[8.0]
+  def change
+    create_table :tag_implications do |t|
+      t.references :tag, null: false, foreign_key: { to_table: :tags }
+      t.references :parent_tag, null: false, foreign_key: { to_table: :tags }
+      t.timestamps
+    end
+  end
+end