Stream ffmpeg -progress during the video encode into an in-memory tracker
(internal/status) so the long-running step is no longer a black box: percent,
fps, speed, and ETA are derived from the source duration and updated ~1/s.
Progress prints to stdout only (no logs.db row) to avoid burying real events.
Expose it over HTTP (internal/server, default :8080, set http_addr to "" to
disable): GET /status returns the live snapshot, the pending input queue, and
recent non-debug events; GET / serves an embedded dashboard that polls /status
every second. The server shuts down on the same SIGINT/SIGTERM context as the
watcher.
