fix: Replace DynamoDB callback with Lambda durable execution callback APIs

- callback_handler: Use context.create_callback() and callback.result()
  instead of DynamoDB-based polling
- template.yaml: Remove CallbackTable, update ECSTaskRole with durable
  execution callback permissions, fix IAM circular dependency using Sub
  with explicit ARN construction
- ECS task now calls SendDurableExecutionCallbackSuccess/Failure directly
- README restructured to match serverless-patterns format

Author: 4D54