Offload Gemini transcript cleanup to a worker thread

[groupthinking]

Move Gemini fallback transcript download artifact deletion off the event loop using a worker thread, while preserving error and cancellation semantics.

[linear-code]

GRV-250

[github-actions]

👋 Thanks for opening this issue! It has been automatically labeled for triage.

[coderabbitai]

⚠️ Possible Duplicate Issue(s)

• perf: transcript Gemini fallback cleans up downloaded video on the event loop #1244

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[groupthinking]

Resolved on `main` by #1245 (merged) — closing as triage hygiene.

Verified in current `origin/main`:

• `src/youtube_extension/services/workflows/transcript_action_workflow.py:926` — `await asyncio.shield(asyncio.to_thread(_cleanup))`
• `src/youtube_extension/services/workflows/transcript_action_workflow.py:1116` — `return await asyncio.to_thread(_download)`

The blocking Gemini cleanup call no longer runs on the event loop. `asyncio.shield` additionally keeps the cleanup from being torn down mid-flight if the awaiting task is cancelled, which is the behaviour you want for a cleanup step.