Offload video detail filesystem lookup off the event loop

[groupthinking]

Offload the uncached video-detail recursive filesystem walk and reads to a worker thread and cap total concurrent walks to prevent event-loop stalling.

[linear-code]

GRV-325

[github-actions]

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

[coderabbitai]

🔗 Related PRs

#1228 - perf: offload video result cache disk I/O to worker threads [merged]
#1237 - perf: offload cache-directory scan off the event loop (#1231) [merged]
#1288 - perf: scan processed-video cache off the event loop [merged]
#1304 - perf: offload video-detail cache read, drop the stat probe [merged]

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