Offload verify_project npm/tsc checks from event loop

[groupthinking]

Wrap npm install, npm build, and npx tsc subprocess.run calls in asyncio.to_thread so project verification doesn’t freeze other requests.

[linear-code]

GRV-248

[github-actions]

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

[coderabbitai]

⚠️ Possible Duplicate Issue(s)

• perf: verify_project blocks the event loop for up to 7 minutes on three subprocess.run calls #1239

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[groupthinking]

Resolved on `main` by #1240 (merged) — closing as triage hygiene.

Verified in current `origin/main`, `src/youtube_extension/backend/deployment_manager.py`:

• L149-150 — `install_result = await asyncio.to_thread(subprocess.run, ...)` (npm install)
• L175-176 — `build_result = await asyncio.to_thread(subprocess.run, ...)` (npm run build)
• L232-233 — `tsc_result = await asyncio.to_thread(subprocess.run, ...)` (tsc --noEmit)

All three long-running toolchain invocations in `verify_project` are now off the event loop, so a verification run no longer stalls concurrent requests for its duration.