Fix Kafka broker connection - use internal Docker port 29092

Fixed two issues with Kafka connectivity:

1. Broker Address: Changed from 'localhost:9092' to 'kafka:29092'
   - localhost:9092 was trying to connect to host machine, not Docker container
   - kafka:29092 is the internal Docker network listener configured in docker-compose
   - This allows the API container to reach Kafka via Docker's internal DNS

2. Kafka Producer Config: Removed invalid 'api_version_auto_discovery_interval_ms'
   - Replaced with 'api_version=(2, 5, 0)' for explicit API version
   - Confluent Kafka 7.5.0 uses API v2.5
   - Prevents unrecognized config warnings

Docker-Compose Kafka Configuration:
  KAFKA_ADVERTISED_LISTENERS:
    - PLAINTEXT://kafka:29092 (for Docker internal communication)
    - PLAINTEXT_HOST://localhost:9092 (for host machine access)

Result: Transactions now successfully published to Kafka!
  POST /transaction → 200 OK with transaction_id
  Message broker available and working

Co-authored-by: Copilot <[email protected]>

api/main.py

@@ -32,8 +32,8 @@ import logging
 SECRET_KEY = "payguard-demo-secret-key-change-in-production"
 ALGORITHM = "HS256"
 
-# Kafka bootstrap server
-KAFKA_BROKER = "localhost:9092"
+# Kafka bootstrap server (use 'kafka:29092' for Docker internal, localhost:9092 for host access)
+KAFKA_BROKER = "kafka:29092"
 KAFKA_TOPIC = "transactions"
 
 # Logging setup
@@ -75,8 +75,8 @@ def get_kafka_producer():
                 value_serializer=lambda v: json.dumps(v).encode('utf-8'),
                 acks='all',
                 retries=3,
-                api_version_auto_discovery_interval_ms=5000,
-                request_timeout_ms=10000
+                request_timeout_ms=10000,
+                api_version=(2, 5, 0)  # Specify Kafka API version
             )
             logger.info(f"[KAFKA PRODUCER] Initialized, broker={KAFKA_BROKER}, topic={KAFKA_TOPIC}")
         except Exception as e: