Extend PatientService::applyDiscount to carry the source rule id/label and
add applyDiscountRule, which computes the rial amount via the engine
(DiscountEngine::computeForRule now public) and records the rule for audit.
updateSession accepts discount_rule_uuid (owner-scoped, takes precedence over
the manual discount; ''/null clears). Verified end-to-end.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>