Implement Queenbee account callbacks

Add Account::QueenbeeIntegration concern (SaaS-only) that bridges
Fizzy's external_account_id to Queenbee's find_by_queenbee_id lookup,
and adapts the bang lifecycle methods to Fizzy's Cancellable module.

Remove dead billing conditional from cancellation mailer — Fizzy is
free-only, there's no charge to stop.

saas/test/models/account/queenbee_integration_test.rb

@@ -0,0 +1,70 @@
+require "test_helper"
+require "queenbee/testing/client"
+
+class Account::QueenbeeIntegrationTest < ActiveSupport::TestCase
+  include Queenbee::Testing::Client
+
+  # Fizzy creates accounts via its own Signup flow (Fizzy → Queenbee),
+  # not via Queenbee pushing to Fizzy, so create_with_dependents is N/A.
+  undef_method :test_client_class_should_respond_to_create_with_dependents
+
+  setup do
+    @account = accounts(:"37s")
+  end
+
+  test "find_by_queenbee_id looks up by external_account_id" do
+    assert_equal @account, Account.find_by_queenbee_id(@account.external_account_id)
+  end
+
+  test "queenbee_id returns external_account_id" do
+    assert_equal @account.external_account_id, @account.queenbee_id
+  end
+
+  test "cancel! creates cancellation attributed to system user" do
+    @account.cancel!
+
+    assert @account.cancelled?
+    assert_equal @account.system_user, @account.cancellation.initiated_by
+  end
+
+  test "cancel! is idempotent" do
+    @account.cancel!
+    assert_no_difference -> { Account::Cancellation.count } do
+      @account.cancel!
+    end
+  end
+
+  test "cancel and reactivate round-trips" do
+    @account.cancel!
+    assert_not @account.active?
+
+    @account.reactivate!
+    @account.reload
+    assert @account.active?
+  end
+
+  test "deactivate! cancels the account" do
+    @account.deactivate!
+    assert @account.cancelled?
+  end
+
+  test "owner_name and owner_email from account owner" do
+    assert_equal users(:jason).name, @account.owner_name
+    assert_equal "jason@37signals.com", @account.owner_email
+  end
+
+  test "comped= is a no-op" do
+    @account.comped = true
+    assert_not @account.comped?
+  end
+
+  test "transferred_ownership! raises NotImplementedError" do
+    assert_raises(NotImplementedError) { @account.transferred_ownership! }
+  end
+
+  test "qb_serializable_hash includes queenbee_id and subscription" do
+    hash = @account.qb_serializable_hash
+    assert_equal @account.external_account_id, hash[:id]
+    assert_equal({ quantity: 1 }, hash[:subscription])
+  end
+end