fix: cancel yt-dlp audio imports reliably when no child process is running

[atomantic]

Summary

cancelYoutubeImport / cancelReferenceAudioImport decided whether a cancel was possible by looking at job.process. That field is transient — it is null while the job is being set up and again during post-processing, because runYtDlp clears it the moment the child exits. A cancel landing in either window returned false, emitted no { type: 'canceled' } frame, and let the import go on to land the audio file and create the Track / uploads record anyway.

• Both jobs now carry a canceled flag; cancel* sets it and only signals killWithEscalation when a child is actually running.
• The kickoff re-checks the flag at each phase boundary (before the download, and again before the file landing / record creation), emitting { type: 'canceled' } and cleaning temp files instead of committing the import.
• The core's canceled outcome and the flag are handled by one shared abort path rather than two branches.
• Cancelling a job that already finished still reports false: the job lingers in the map until closeJobAfterDelay evicts it, so the kickoff marks it done and cancel* requires a running status.
• Both modules export __testing = { importJobs }, matching videoDownload.js, so the cancel path is unit-testable at all.

Test plan

• server/services/trackYoutubeImport.test.js and server/services/roundReferenceAudioImport.test.js gain cancellation coverage: unknown job id, cancel with no child spawned yet, cancel signalling a running child plus a rejected second cancel, cancel of a finished-but-not-yet-evicted job, and a kickoff cancelled after the download finished (asserts canceled is broadcast and importUploadedTrack / createTrack / importFileToUploads are never called).
• Verified the new tests fail against the pre-fix sources (6 failures) and pass after.
• cd server && npx vitest run services/trackYoutubeImport.test.js services/roundReferenceAudioImport.test.js services/ytdlpAudioImport.test.js services/videoDownload.test.js routes/tracks.test.js routes/rounds.test.js — all green.

Closes #6013