Update JS dependencies and add CDK stack unit tests

[bermannoah]

Summary

• Update non-CDK JS dependencies (typescript 3.7 → 4.9, ts-node 8 → 9, @types/node 10 → 18 LTS, source-map-support patch bump) in both root and test/workflow
• Upgrade all @aws-cdk/* and aws-cdk packages from 1.75.0 → 1.204.0 (latest 1.x) in both root and test/workflow
• Add jest@27, ts-jest@27, @aws-cdk/assert@1.204.0 and 12 CDK stack unit tests covering:
  • Lambda runtime/count (PROVIDED_AL2, bootstrap handler)
  • DynamoDB TTL and stream config
  • SQS queues and dead-letter queue wiring
  • Step Functions state machines
  • API Gateway resources when enableAPI is true/false
  • SNS topic subscription via alertTopicARN
  • Asset path validation error message
• Add js-build CI job running npm ci, npm run prepare (tsc), and npm test

CDK v2 migration is tracked separately.

Test plan

• js-build CI job passes (npm ci + tsc + jest)
• Go lint and test jobs unaffected

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