Add Filters::Stage

This adds a filter for selecting application forms according to their
stage.

app/lib/filters/stage.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+class Filters::Stage < Filters::Base
+  def apply
+    return scope if stage.empty?
+
+    scope.where(stage:)
+  end
+
+  private
+
+  def stage
+    Array(params[:stage]).compact_blank
+  end
+end

spec/lib/filters/stage_spec.rb

@@ -0,0 +1,34 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+RSpec.describe Filters::Stage do
+  subject(:filtered_scope) { described_class.apply(scope:, params:) }
+
+  context "the params includes a stage" do
+    let(:params) { { stage: "not_started" } }
+    let(:scope) { ApplicationForm.all }
+
+    let!(:included) { create(:application_form, :not_started_stage) }
+    let!(:excluded) { create(:application_form, :assessment_stage) }
+
+    it { is_expected.to contain_exactly(included) }
+  end
+
+  context "the params include multiple stage" do
+    let(:params) { { stage: %w[not_started assessment] } }
+    let(:scope) { ApplicationForm.all }
+
+    let!(:included) { create(:application_form, :not_started_stage) }
+    let!(:excluded) { create(:application_form, :completed_stage) }
+
+    it { is_expected.to contain_exactly(included) }
+  end
+
+  context "the params don't include stage" do
+    let(:params) { {} }
+    let(:scope) { double }
+
+    it { is_expected.to eq(scope) }
+  end
+end