What: Make the LibreTranslate HTTP call in output_translate() asynchronous so it does not block the ASIO event loop thread.
Why: Currently every translation tick blocks the single ASIO thread for the round-trip duration. The 10s CURL timeout bounds the worst case, but every tick still introduces latency — the 10s poll timer drifts by the actual LibreTranslate response time.
Pros: Timer loop stays fully responsive during translation; Ctrl+C processes immediately; window-exit check is never delayed by a slow LibreTranslate server.
Cons: Adds complexity (thread pool or detached thread + result posting back to ASIO context); needs care around _output_file lifetime and mutex when posting results across threads.
Context: This was explicitly listed as a Non-Goal in the analyze-and-optimize design doc (D6 decision). The CURL timeout mitigation was chosen as the minimal "boring" fix. Async translation is the correct long-term direction for a real-time caption tool. Starting point: wrap the CURL call in asio::post(thread_pool, ...) and post the translated result back with asio::post(io_context, ...).
Depends on: analyze-and-optimize change (adds the CURL timeout baseline and the single-call parse_captions() refactor).