feat(poi): multi-container DLStreamer scaling and benchmark reliability

[sainijit]

• Fix DLStreamer config generation to distribute cameras across both lp-video containers (round-robin)
• Improve alert thumbnail saving and stale alert filtering
• Add E2E latency measurement from alerts API
• Better single-run benchmark handling with early exit on alert

PR Checklist

• Added label to the Pull Request for easier discoverability and search
• Commit Message meets guidelines as indicated in the URL https://github.com/intel-retail/performance-tools/blob/main/CONTRIBUTING.md
• Every commit is a single defect fix and does not mix feature addition or changes
• Unit Tests have been added for new changes
• Updated Documentation as relevant to the changes
• All commented code has been removed
• If you've added a dependency, you've ensured license is compatible with repository license and clearly outlined the added dependency.
• PR change contains code related to security
• PR introduces changes that breaks compatibility with other modules (If YES, please provide details below)

What are you changing?

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close: #issue_number

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[Copilot]

Pull request overview

This PR updates the POI stream-density benchmark flow to better support multiple DLStreamer containers and reduce benchmark wait time.

Changes:

• Distributes generated DLStreamer pipelines across lp-video and lp-video-2.
• Reworks scaling to start extra camera services and recreate DLStreamer containers with readiness polling.
• Adds parallel scene cleanup/import and service recreation behavior.

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