feat(payment-gated-subs): Delegate downgrade logic to its own service

[osmarluz]

Context

Subscriptions::PlanUpgradeService already owns the upgrade flow, but downgrades stayed inline in Subscriptions::CreateService. This PR brings the two paths into symmetry ahead of the payment-gating work for plan changes (Milestone 3). Pure refactor — no behavior change.

Description

• Subscriptions::PlanDowngradeService (new) owns the downgrade flow, mirroring Subscriptions::PlanUpgradeService. Returns result.subscription = current_subscription — the load-bearing difference from upgrades, which return the new subscription.

• Subscriptions::CreateService#downgrade_subscription collapses to a one-line delegation to the new service.

[ancorcruz]

Could we add a case covering the rescue ActiveRecord::RecordInvalid branch? in spec/services/subscriptions/plan_downgrade_service_spec.rb. Something like:

context "when creating the next subscription fails validation" do
  before do
    allow_any_instance_of(Subscription)
      .to receive(:save!)
      .and_raise(ActiveRecord::RecordInvalid.new(Subscription.new))
  end

  it "returns a record validation failure" do
    expect(result).not_to be_success
    expect(result.error).to be_a(BaseService::ValidationFailure)
  end
end

Right now the rescue branch is uncovered for a freshly-extracted service — would rather pin it down.