feat: enable redis sentinel for automatic failovers and high availability

[felixschmetz]

Summary by cubic

Enable Redis high availability via Sentinel with automatic failover. Also adds a Terraform stack for terratest (S3, ECS, CloudWatch logs, security group, IAM).

• New Features

  • Redis client auto-detects Sentinel vs direct mode, toggled by REDIS_SENTINEL_ENABLED; Sentinel uses service name "mymaster" on port 26379 with timeouts, keepalive, and retries.
  • Direct mode now connects to "<REDIS_HOST>-master" to match StatefulSet naming.
  • Added terratest/main.tf provisioning S3 bucket, ECS cluster, CloudWatch log group, ECS task security group, and ECS task execution role.

• Migration

  • To use Sentinel, set REDIS_SENTINEL_ENABLED=true and ensure the Sentinel service is reachable at host:26379 with master name "mymaster".
  • In non-Sentinel setups, set REDIS_HOST without the "-master" suffix; the client appends it automatically.

^{Written for commit b9829a9. Summary will update automatically on new commits.}