⚡ Bolt: Offload CPU-bound bcrypt operations to threadpool

[benpiper]

💡 What: Refactored verify_password and get_password_hash in backend/auth.py to be async functions that offload bcrypt hashing to a threadpool via Starlette's run_in_threadpool. Updated callers in backend/main.py.

🎯 Why: Hashing and verifying passwords using bcrypt takes hundreds of milliseconds and is CPU-bound. In FastAPI, blocking the asyncio event loop with CPU-bound synchronous code halts all other concurrent requests, tanking throughput under load. Offloading these to a threadpool preserves event loop responsiveness.

📊 Impact: Significantly improves concurrency and prevents a single login or registration request from blocking the entire server for other users.

🔬 Measurement: Tested via unit tests (cd backend && uv run pytest tests/) and by ensuring lint passes (cd backend && uv run ruff check .). Concurrency testing would show the API remaining responsive to requests while passwords are being hashed.

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PR created automatically by Jules for task 13785528567249135508 started by @benpiper

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