Per-request deadline middleware.

State: #{after_ms => Ms}. If the rest of the pipeline does not return within the deadline, the worker is killed and a 504 is emitted instead. A handler crash maps to 500.

The deadline is enforced by running the downstream call in a spawned, monitored process. Body chunks (livery_body:read/2) are delivered to the request process, not this spawned child, so a handler that streams its request body will not see those chunks under this middleware. Pair livery_timeout with a body-buffering middleware in front of it, or apply it only to routes whose handlers do not stream input.