Activate background report processing by default

Many report specs are still testing the old behaviour. We need to
migrate them to background reports. Some tests may be better as unit
tests instead of system tests.

db/migrate/20231129222807_enable_feature_backround_reports.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class EnableFeatureBackroundReports < ActiveRecord::Migration[7.0]
+  def up
+    Flipper.enable("background_reports")
+  end
+end

lib/open_food_network/feature_toggle.rb

@@ -10,6 +10,17 @@ module OpenFoodNetwork
     # Please add your new feature here to appear in the Flipper UI.
     # We way move this to a YAML file when it becomes too awkward.
     # **WARNING:** Features not in this list will be removed.
+    #
+    # Once the feature is ready for general production use,
+    # copy the feature declaration to ACTIVE_BY_DEFAULT below and
+    # activate it for all instances with a migration:
+    #
+    #   ./bin/rails generate migration EnableFeatureDragonMode
+    #
+    # Replace the `change` method with an `up` method and add this line:
+    #
+    #   Flipper.enable("dragon_mode")
+    #
     CURRENT_FEATURES = {
       "admin_style_v3" => <<~DESC,
         Test the work-in-progress design updates.
@@ -38,10 +49,23 @@ module OpenFoodNetwork
       DESC
     }.freeze
 
+    # Features you would like to be enabled to start with.
+    #
+    # Copy features here that were activated in a migration so that new
+    # instances, development and test environments have the feature active.
+    ACTIVE_BY_DEFAULT = {
+      "background_reports" => <<~DESC,
+        Generate reports in a background process to limit memory consumption.
+      DESC
+    }.freeze
+
     def self.setup!
       CURRENT_FEATURES.each_key do |name|
         feature = Flipper.feature(name)
-        feature.add unless feature.exist?
+        unless feature.exist?
+          feature.add
+          feature.enable if ACTIVE_BY_DEFAULT[name]
+        end
       end
 
       Flipper.features.each do |feature|

spec/base_spec_helper.rb

@@ -88,6 +88,33 @@ RSpec.configure do |config|
     expectations.syntax = :expect
   end
 
+  # Reset all feature toggles to prevent leaking.
+  config.before(:suite) do
+    Flipper.features.each(&:remove)
+    OpenFoodNetwork::FeatureToggle.setup!
+  end
+
+  config.before(:each) do |example|
+    Flipper.disable(:background_reports) if example.file_path.in?(
+      [
+        # rubocop:disable Layout/LineLength
+        "./spec/controllers/admin/reports_controller_spec.rb",
+        "./spec/system/admin/reports/enterprise_fee_summaries_spec.rb",
+        "./spec/system/admin/reports/enterprise_summary_fees/enterprise_summary_fee_with_tax_report_by_order_spec.rb",
+        "./spec/system/admin/reports/enterprise_summary_fees/enterprise_summary_fee_with_tax_report_by_producer_spec.rb",
+        "./spec/system/admin/reports/orders_and_fulfillment_spec.rb",
+        "./spec/system/admin/reports/packing_report_spec.rb",
+        "./spec/system/admin/reports/payments_report_spec.rb",
+        "./spec/system/admin/reports/revenues_by_hub_spec.rb",
+        "./spec/system/admin/reports/sales_tax/sales_tax_totals_by_order_spec.rb",
+        "./spec/system/admin/reports/sales_tax/sales_tax_totals_by_producer_spec.rb",
+        "./spec/system/admin/reports/users_and_enterprises_spec.rb",
+        "./spec/system/admin/reports_spec.rb",
+        # rubocop:enable Layout/LineLength
+      ]
+    )
+  end
+
   config.before(:each, :feature) do |example|
     feature = example.metadata[:feature].to_s