Analyze batch videos concurrently with batch_size bound

[groupthinking]

Make process_batch_videos fan out analysis within each batch using asyncio.gather, preserving failure isolation, ordering, and cancellation propagation.

[linear-code]

GRV-224

[github-actions]

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

[coderabbitai]

⚠️ Possible Duplicate Issue(s)

• perf(cloud-ai): process_batch_videos analyses each batch sequentially, making batch_size inert #1187

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[groupthinking]

Already resolved on main — closing as complete

This was filed on 2026-08-01, the same day the fix merged. The tracking issue was never
linked to the PR that implemented it, so it stayed open. The work described here is
already shipped; leaving it open risks somebody re-implementing it.

Fix: 8fd1d60d0 (#1188) — perf(cloud-ai): analyse each batch concurrently in process_batch_videos
Code: src/youtube_extension/backend/cloud_ai_routes.py

process_batch_videos fans each batch out concurrently while remaining bounded by batch_size, so one failed video no longer aborts the batch.

Verification performed

1. Commit is on main — git merge-base --is-ancestor 8fd1d60d0 origin/main exits 0.

2. Mechanism present in current origin/main — confirmed by reading
   git show origin/main:src/youtube_extension/backend/cloud_ai_routes.py, not just the merge record.

3. Regression suite shipped with the fix passes:

   pytest tests/unit/test_cloud_ai_routes_batch.py
   # 7 passed
   

No further work required.

_{Verified by an automated triage pass. The whole async-fix cluster
(#1191, #1192, #1198, #1199, #1201, #1206) was re-run together: 442 passed, 0 failed.}