GHSA-mjgf-xj26-9qf9: pay-rails/pay: non-constant-time HMAC comparison in Paddle Billing webhook signature verifier

Pay::Webhooks::PaddleBillingController#valid_signature? (app/controllers/pay/webhooks/paddle_billing_controller.rb) verifies the Paddle Billing webhook signature by computing OpenSSL::HMAC.hexdigest(...) and comparing it to the attacker-supplied header value using Ruby’s String#==. Ruby’s == is non-constant-time — it returns as soon as the first byte mismatches — and exposes a per-byte timing side channel on the webhook signature verification path. The canonical mitigation is to use a constant-time primitive (OpenSSL.fixed_length_secure_compare / ActiveSupport::SecurityUtils.secure_compare).