Add Payment module: contracts, payments/refunds tables, KBZ gateway, factory, orchestrator (T5.1-T5.7)

- T5.1 PaymentGatewayInterface, DTOs, PaymentMethod/PaymentStatus/RefundStatus enums
- T5.2 payments/refunds tables, models, factories
- T5.3-T5.5 KbzMiniAppGateway: initiate()/verify()/refund(), ported KBZ signing scheme,
  wired refund_amount through for partial refunds, mTLS options for refund
- T5.6 PaymentGatewayFactory resolving gateways by PaymentMethod
- T5.7 PaymentService orchestrator delegating to the resolved gateway

app-modules/payment/database/factories/RefundFactory.php

@@ -0,0 +1,52 @@
+<?php
+
+namespace Modules\Payment\Database\Factories;
+
+use Illuminate\Database\Eloquent\Factories\Factory;
+use Modules\Payment\Enums\RefundStatus;
+use Modules\Payment\Models\Payment;
+use Modules\Payment\Models\Refund;
+
+/**
+ * @extends Factory<Refund>
+ */
+class RefundFactory extends Factory
+{
+    protected $model = Refund::class;
+
+    /**
+     * Define the model's default state.
+     *
+     * @return array<string, mixed>
+     */
+    public function definition(): array
+    {
+        return [
+            'payment_id' => Payment::factory()->completed(),
+            'status' => RefundStatus::Pending,
+            'amount' => $this->faker->randomFloat(2, 1000, 50000),
+            'reason' => $this->faker->sentence(),
+            'gateway_refund_id' => null,
+            'gateway_payload' => null,
+            'requested_by' => null,
+            'requested_at' => now(),
+            'completed_at' => null,
+        ];
+    }
+
+    public function completed(): static
+    {
+        return $this->state(fn (array $attributes): array => [
+            'status' => RefundStatus::Completed,
+            'completed_at' => now(),
+        ]);
+    }
+
+    public function failed(): static
+    {
+        return $this->state(fn (array $attributes): array => [
+            'status' => RefundStatus::Failed,
+            'completed_at' => now(),
+        ]);
+    }
+}