refactor: remove batch mode, simplify to API-only deployment

Removes dual-mode deployment complexity, focusing on REST API service only.

Changes:
- Removed batch mode from docker-compose.yml (now single ai-trader service)
- Deleted scripts/test_batch_mode.sh validation script
- Renamed entrypoint-api.sh to entrypoint.sh (now default)
- Simplified Dockerfile (single entrypoint, removed CMD)
- Updated validation scripts to use 'ai-trader' service name
- Updated documentation (README.md, TESTING_GUIDE.md, CHANGELOG.md)

Benefits:
- Eliminates port conflicts between batch and API services
- Simpler configuration and deployment
- API-first architecture aligned with Windmill integration
- Reduced maintenance complexity

Breaking Changes:
- Batch mode no longer available
- All simulations must use REST API endpoints

scripts/test_api_endpoints.sh

@@ -25,7 +25,7 @@ echo "Checking if API is accessible..."
 if ! curl -f "$API_BASE_URL/health" &> /dev/null; then
     echo -e "${RED}✗${NC} API is not accessible at $API_BASE_URL"
     echo "Make sure the container is running:"
-    echo "  docker-compose up -d ai-trader-api"
+    echo "  docker-compose up -d ai-trader"
     exit 1
 fi
 echo -e "${GREEN}✓${NC} API is accessible"