feat(discount): add DiscountRule entity and session discount-rule audit columns

New Discount domain: DiscountRule (generic per-tenant rule with 6 types,
priority, combinable, validity window, and per-type target fields) plus its
repository. PatientSession gains applied_discount_rule_id/label audit columns
and setDiscount() now records the source rule. Migration creates the table
and columns.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

migrations/Version20260717081632.php

@@ -0,0 +1,31 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DoctrineMigrations;
+
+use Doctrine\DBAL\Schema\Schema;
+use Doctrine\Migrations\AbstractMigration;
+
+/**
+ * Auto-generated Migration: Please modify to your needs!
+ */
+final class Version20260717081632 extends AbstractMigration
+{
+    public function getDescription(): string
+    {
+        return 'Add discount_rules table + applied_discount_rule audit columns on patient_sessions';
+    }
+
+    public function up(Schema $schema): void
+    {
+        $this->addSql('CREATE TABLE discount_rules (id INT AUTO_INCREMENT NOT NULL, uuid VARCHAR(36) NOT NULL, owner_type VARCHAR(10) NOT NULL, owner_id INT NOT NULL, name VARCHAR(120) NOT NULL, type VARCHAR(20) NOT NULL, discount_type VARCHAR(10) NOT NULL, value INT NOT NULL, priority INT NOT NULL, combinable TINYINT DEFAULT 0 NOT NULL, active TINYINT DEFAULT 1 NOT NULL, valid_from INT DEFAULT NULL, valid_to INT DEFAULT NULL, target_tag_id INT DEFAULT NULL, target_record_id INT DEFAULT NULL, target_service_item_id INT DEFAULT NULL, min_amount_rials INT DEFAULT NULL, min_visit_count INT DEFAULT NULL, occasion_kind VARCHAR(20) DEFAULT NULL, created_at INT NOT NULL, updated_at INT NOT NULL, UNIQUE INDEX UNIQ_A2B041ABD17F50A6 (uuid), INDEX idx_discount_rules_owner (owner_type, owner_id, active), PRIMARY KEY (id)) DEFAULT CHARACTER SET utf8mb4');
+        $this->addSql('ALTER TABLE patient_sessions ADD applied_discount_rule_id INT DEFAULT NULL, ADD applied_discount_rule_label VARCHAR(120) DEFAULT NULL');
+    }
+
+    public function down(Schema $schema): void
+    {
+        $this->addSql('DROP TABLE discount_rules');
+        $this->addSql('ALTER TABLE patient_sessions DROP applied_discount_rule_id, DROP applied_discount_rule_label');
+    }
+}