feat: Implement payment service with Stripe integration

[rezadrian01]

Summary

• Full payment service implementation (port 8084) with Stripe PaymentIntents integration
• Kafka consumer subscribes to order.created → creates Stripe PaymentIntent → stores Payment record
• Webhook handler processes payment_intent.succeeded/payment_failed/processing events and publishes payment.created/completed/failed Kafka events
• Redis cache-aside for payment lookups (1h TTL)
• Fixed pre-existing docker-compose bug: kafka, zookeeper, and kafka-ui were missing networks: auron-network, making them unreachable from all microservices

Changes

New service — services/payment-service/

• Domain layer: Payment entity, PaymentStatus, OrderCreatedEvent, repository/service/cache/events/stripe client interfaces, 5 sentinel errors
• DB migration: payments table with status CHECK constraint, amount > 0 CHECK, unique index on order_id
• Repository: 5 GORM methods; UpdatePaymentStatus and UpdateStripeIDs as separate operations
• Cache: payment:<uuid> key, 1h TTL
• Kafka publisher: writes to payment.created, payment.completed, payment.failed
• Kafka consumer: reads order.created (group payment-service); idempotent via GetPaymentByOrderID check
• Stripe client: wraps stripe-go/v76; converts amount to cents; passes payment_id/order_id/user_id as Stripe metadata
• Service: client_secret only travels via payment.created Kafka event, never HTTP; webhook verification bypassed when STRIPE_WEBHOOK_SECRET is empty (dev)
• Raw body middleware: captures request bytes before binding for Stripe signature verification
• cmd bootstrap: full wiring with context.WithCancel for clean consumer shutdown on SIGTERM

docker-compose.yml

• Added REDIS_URL, KAFKA_BROKERS, STRIPE_SECRET_KEY, STRIPE_WEBHOOK_SECRET to payment-service
• Added kafka: condition: service_healthy to payment-service.depends_on
• Added networks: - auron-network to zookeeper, kafka, kafka-ui (bug fix)

Test plan

• docker-compose up payment-service payments-db redis kafka starts without errors
• GET /health returns {"status":"healthy","service":"payment-service"}
• POST /api/payments/webhook/stripe with a valid Stripe test event returns 200
• GET /api/payments/:id with a valid auth token returns the payment or 404
• Creating an order triggers order.created → payment-service creates a PaymentIntent and publishes payment.created
• Stripe payment_intent.succeeded webhook updates payment status to completed and publishes payment.completed

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