fix: isolate transcription results by media

[FernandoAbishai]

Phase 7D.1 P1 corrective

• Isolates asynchronous transcription runs by monotonic generation and expected media path.
• Prevents stale transcript, active-panel, message/log/progress, error, finally, and job-id writes after media/project replacement.
• Preserves cancellation, retry, current-run completion, full-video/short workflow intent, Electron picker cancellation, and failed-upload behavior.
• Adds deterministic lifecycle regression coverage for stale/current results, polling UI, cleanup, errors, job-id races, transitions, restore, retry, and panel intent.
• No backend, project schema, AI clip workspace, release, or version changes.

Verification

• All 21 frontend smoke scripts: PASS
• Backend smoke: PASS (121 tests, loopback permission enabled)
• npm run lint: PASS
• npm run build:frontend: PASS
• git diff --check: PASS

Do not merge.

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