Fix broken background image after removal (#2817)

ActiveStorage::Record and ApplicationRecord used separate connection
pools (from separate `connects_to` calls). This meant `belongs_to
touch: true` on attachments silently failed — the card couldn't join the
attachment's transaction to receive the deferred touch, so fragment
caches were never invalidated after image removal.

Fix by delegating `connection_pool` from ActiveStorage::Record to
ApplicationRecord so they share a single pool.

test/models/storage/no_reuse_test.rb

@@ -57,6 +57,32 @@ class Storage::NoReuseTest < ActiveSupport::TestCase
     ).count
   end
 
+  test "purge touches the record to invalidate cache" do
+    card = @board.cards.create!(title: "Card", creator: users(:david))
+    card.image.attach(io: file_fixture("moon.jpg").open, filename: "moon.jpg", content_type: "image/jpeg")
+
+    original_updated_at = card.reload.updated_at
+
+    travel 1.second do
+      card.image.purge
+    end
+
+    assert card.reload.updated_at > original_updated_at
+  end
+
+  test "purge_later touches the record to invalidate cache" do
+    card = @board.cards.create!(title: "Card", creator: users(:david))
+    card.image.attach(io: file_fixture("moon.jpg").open, filename: "moon.jpg", content_type: "image/jpeg")
+
+    original_updated_at = card.reload.updated_at
+
+    travel 1.second do
+      card.image.purge_later
+    end
+
+    assert card.reload.updated_at > original_updated_at
+  end
+
   test "purge_later does not purge blob when still attached elsewhere" do
     file = file_fixture("moon.jpg")
     blob = ActiveStorage::Blob.create_and_upload! \