Kafka producer integration for transaction event streaming

[Teboho66]

Description

The A3 architecture specifies Apache Kafka as the event backbone. This
issue adds a Kafka producer to the transaction submission endpoint so
that every accepted transaction is published to transactions.raw.

Tasks

• Add confluent-kafka to requirements.txt
• Create infrastructure/kafka_producer.py
• Publish to transactions.raw after TransactionService.submit_transaction()
• Add Kafka to docker-compose.yml (Kafka + Zookeeper)
• Add integration test verifying message is produced

Acceptance Criteria

• POST /api/transactions publishes event to transactions.raw
• Message contains tokenised payload (no raw PII — BR-T2)
• If Kafka is unavailable, the API still accepts the transaction
  (graceful degradation)