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/migrations/2026_08_08_100000_create_payments_table.php

@@ -0,0 +1,40 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * One row per payment attempt against a Booking — a Booking can have
+     * more than one row here if an earlier attempt failed and the customer
+     * retried (domain.md §1, §6).
+     */
+    public function up(): void
+    {
+        Schema::create('payments', function (Blueprint $table) {
+            $table->id();
+            $table->foreignId('booking_id')->constrained('bookings')->cascadeOnDelete();
+            $table->string('gateway');
+            $table->string('status')->default('pending');
+            $table->decimal('amount', 10, 2);
+            $table->string('currency')->default('MMK');
+            $table->string('gateway_transaction_id')->nullable()->index();
+            $table->jsonb('gateway_payload')->nullable();
+            $table->timestamp('initiated_at')->nullable();
+            $table->timestamp('completed_at')->nullable();
+            $table->timestamps();
+
+            $table->index(['booking_id', 'status']);
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::dropIfExists('payments');
+    }
+};

app-modules/payment/database/migrations/2026_08_08_100100_create_refunds_table.php

@@ -0,0 +1,40 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * A Refund reverses a specific successful Payment, not the Booking
+     * directly — a Payment can have more than one Refund row for partial
+     * refunds (domain.md §1, §6).
+     */
+    public function up(): void
+    {
+        Schema::create('refunds', function (Blueprint $table) {
+            $table->id();
+            $table->foreignId('payment_id')->constrained('payments')->cascadeOnDelete();
+            $table->string('status')->default('pending');
+            $table->decimal('amount', 10, 2);
+            $table->text('reason');
+            $table->string('gateway_refund_id')->nullable()->index();
+            $table->jsonb('gateway_payload')->nullable();
+            $table->foreignId('requested_by')->nullable()->constrained('users')->nullOnDelete();
+            $table->timestamp('requested_at')->nullable();
+            $table->timestamp('completed_at')->nullable();
+            $table->timestamps();
+
+            $table->index(['payment_id', 'status']);
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::dropIfExists('refunds');
+    }
+};