The cart API returns 503: Fargate tasks die before they even start

Atlas Panier is an online grocery store in Casablanca (2-hour delivery). The panier-api API runs on ECS Fargate (cluster prod-atlas, 4 tasks behind an ALB).

Last night, as part of a “network hardening” effort, the team:

• moved the service to two private subnets (10.42.11.0/24 and 10.42.12.0/24) and disabled the tasks' public IP;
• deleted the old NAT gateway, considered too expensive (“we'll go through endpoints”).

This morning, a routine deployment (panier-api:2026.10.18-3) replaced the tasks. Since 08:05, the site shows “Service unavailable” when the cart is checked out. The morning's orders are blocked.

You are on your workstation with the AWS CLI v2 (operations profile, region eu-west-3).