[P2] Video export tmpdir leaks if the client disconnects mid-stream

[eboudrant]

Priority: P2 · Review finding: #11

src/handler.js /api/scans/:id/video cleans the tmpdir on the stream's finish and error events, but not on close — if the client disconnects mid-download, finish never fires and the tmpdir (with the rendered mp4) is leaked.

Fix: also hook res.on('close', ...) (idempotent cleanup), or use stream.pipeline with a finally.