⚡ Bolt: Replace blocking time.sleep with asyncio.sleep in get_frame_image

[benpiper]

What: Replaced synchronous time.sleep with await asyncio.sleep in the async def get_frame_image endpoint in backend/main.py.
Why: time.sleep blocks the entire FastAPI event loop, crippling performance and severely degrading overall application throughput for concurrent requests.
Impact: Resolves a critical backend concurrency bottleneck, enabling concurrent processing and drastically improving server responsiveness.
Measurement: Run load testing or concurrent requests to get_frame_image and verify the server remains responsive without blocking other incoming requests.

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PR created automatically by Jules for task 2503741357199439910 started by @benpiper

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