⚡ Bolt: Prevent event loop blocking in async routes

[benpiper]

💡 What: Wrapped CPU-bound bcrypt hashing/verification functions (get_password_hash and verify_password) in backend/auth.py with starlette.concurrency.run_in_threadpool, making them asynchronous, and updated all callers in backend/main.py and backend/auth.py to await them. Also replaced a synchronous time.sleep(1) loop in get_frame_image with await asyncio.sleep(1).
🎯 Why: In FastAPI, running CPU-bound operations (like bcrypt) or synchronous blocking calls (time.sleep) directly inside async def routes blocks the entire event loop. This starves the server and prevents it from handling concurrent requests effectively.
📊 Impact: Significantly improves server concurrency and responsiveness, eliminating bottlenecks during user authentication and image polling.
🔬 Measurement: Verified functionality by passing all existing backend tests (pytest tests/). Performance improvements can be measured under load testing by observing the API's concurrent request handling capabilities.

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PR created automatically by Jules for task 8965965160403963842 started by @benpiper

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