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

config/services.php

@@ -36,6 +36,7 @@ return [
     ],
 
     'kbz' => [
+        'app_id' => env('KBZ_APP_ID'),
         'merchant_code' => env('KBZ_MERCHANT_CODE'),
         'merchant_key' => env('KBZ_MERCHANT_KEY'),
         'base_url' => env('KBZ_BASE_URL'),