外部タグを分離 (#419) (#420)

**本番 DB のバックアップをかならずすること**

Reviewed-on: #420
Co-authored-by: miteruzo <miteruzo@naver.com>

backend/spec/services/post_create_plan_spec.rb

@@ -50,6 +50,19 @@ RSpec.describe PostCreatePlan do
     expect(plan[:video_ms]).to eq(60_000)
   end
 
+  it 'rejects direct external tag input without creating internal records' do
+    create(:external_tag, name: 'reserved')
+    counts = [Tag.count, TagName.count, ExternalTag.count]
+
+    expect {
+      described_class.new(attributes: { tags: 'NiCo:reserved' }).build!
+    }.to raise_error(ActiveRecord::RecordInvalid) { |error|
+      expect(error.record.errors[:tags]).to be_present
+    }
+
+    expect([Tag.count, TagName.count, ExternalTag.count]).to eq(counts)
+  end
+
   it 'validates a new tag name without persisting it' do
     long_name = 'a' * 256
     counts = [TagName.count, Tag.count]