feat(discount): apply a discount rule to a session with audit

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>

src/Discount/Service/DiscountEngine.php

@@ -36,14 +36,7 @@ class DiscountEngine
 
         $out = [];
         foreach ($rules as $rule) {
-            if (!$this->withinValidity($rule, $now)) {
-                continue;
-            }
-            $base = $this->applicableBase($rule, $session, $final, $now);
-            if ($base === null) {
-                continue;
-            }
-            $rials = min($this->computeRials($rule, $base), $remaining);
+            $rials = $this->computeForRule($session, $rule, $now, $remaining);
             if ($rials <= 0) {
                 continue;
             }
@@ -61,6 +54,24 @@ class DiscountEngine
         return $out;
     }
 
+    /**
+     * مبلغ تخفیف ریالیِ یک قانون برای یک پرونده؛ 0 اگر برقرار نباشد.
+     * $remaining را ندهی، از خود session محاسبه می‌شود.
+     */
+    public function computeForRule(PatientSession $session, DiscountRule $rule, ?int $now = null, ?int $remaining = null): int
+    {
+        $now ??= time();
+        $remaining ??= max(0, $session->getFinalPriceRials() - $session->getPaidTotalRials());
+        if (!$this->withinValidity($rule, $now)) {
+            return 0;
+        }
+        $base = $this->applicableBase($rule, $session, $session->getFinalPriceRials(), $now);
+        if ($base === null) {
+            return 0;
+        }
+        return min($this->computeRials($rule, $base), $remaining);
+    }
+
     private function withinValidity(DiscountRule $rule, int $now): bool
     {
         if ($rule->getValidFrom() !== null && $now < $rule->getValidFrom()) {