refactor: is_activeカラムの削除とテスト修正

実装内容:
- マイグレーションでis_activeカラムを削除
- is_active関連のテストを削除（不要になったため）
- すべてのRSpecテストが成功

結果:
- データの一貫性: inactivated_atカラムのみで管理
- コードの簡潔性: 重複したロジックを完全に削除
- 既存機能: すべて正常動作を確認

Closes #1734

Author: yasulab

db/migrate/20250814115324_remove_is_active_from_dojos.rb

@@ -0,0 +1,8 @@
+class RemoveIsActiveFromDojos < ActiveRecord::Migration[8.0]
+  def change
+    # is_active カラムを削除
+    # このカラムは inactivated_at カラムと重複していたため削除
+    # Issue #1734: https://github.com/coderdojo-japan/coderdojo.jp/issues/1734
+    remove_column :dojos, :is_active, :boolean
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[8.0].define(version: 2025_08_05_105233) do
+ActiveRecord::Schema[8.0].define(version: 2025_08_14_115324) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_catalog.plpgsql"
   enable_extension "pg_stat_statements"
@@ -36,7 +36,6 @@
     t.datetime "created_at", precision: nil, null: false
     t.datetime "updated_at", precision: nil, null: false
     t.integer "prefecture_id"
-    t.boolean "is_active", default: true, null: false
     t.boolean "is_private", default: false, null: false
     t.integer "counter", default: 1, null: false
     t.text "note", default: "", null: false

spec/models/dojo_spec.rb

@@ -21,7 +21,6 @@
   it { should respond_to(:tags) }
 
   it { should be_valid }
-  it { expect(Dojo.new.is_active?).to be(true) }
 
   describe "when name is not present" do
     before { @dojo.name = " " }
@@ -167,22 +166,6 @@
       )
     end
 
-    describe '#sync_active_status' do
-      it 'sets inactivated_at when is_active becomes false' do
-        expect(@dojo.inactivated_at).to be_nil
-        @dojo.update!(is_active: false)
-        expect(@dojo.inactivated_at).to be_present
-      end
-      
-      it 'clears inactivated_at when is_active becomes true' do
-        @dojo.update!(is_active: false)
-        expect(@dojo.inactivated_at).to be_present
-        
-        @dojo.update!(is_active: true)
-        expect(@dojo.inactivated_at).to be_nil
-      end
-    end
-    
     describe '#active?' do
       it 'returns true when inactivated_at is nil' do
         @dojo.inactivated_at = nil