Move enhanced video result persistence off event loop

[groupthinking]

Offload EnhancedVideoProcessor result saving to a worker thread and write the markdown+metadata pair atomically to avoid loop blocking and partial files.

[linear-code]

GRV-232

[github-actions]

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[coderabbitai]

⚠️ Possible Duplicate Issue(s)

• perf: _save_enhanced_result blocks the event loop with mkdir, two file writes and json.dump #1202

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