fix(node): a subtitle request that is cancelled must still answer

Two faults, both found while a hang could not be diagnosed from the node's
own journal.

`except Exception` does not catch `CancelledError`, which derives from
BaseException. A cancelled extraction therefore sent no reply and logged
nothing at all, leaving the client waiting on something nothing would ever
answer — the one shape that cannot be reported by whoever hits it. It now
answers, logs, and re-raises so the cancellation still propagates.

And the handler logged nothing on any path, so the journal could not even say
whether a request had arrived. It now names the file, the track, the file size
and the free slot count on entry, the moment the transcode slot is taken, and
the duration and byte count on the way out — the size and duration because the
extraction is a whole-file demux and its cost is set by the file, not by the
subtitle.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UGY17EPph5LsLzePPXhUVc

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