⚡ Bolt: Offload bcrypt operations to threadpool in async routes

[benpiper]

💡 What

Offloaded CPU-bound bcrypt operations (bcrypt.checkpw and bcrypt.hashpw) to a background thread using starlette.concurrency.run_in_threadpool. Converted verify_password and get_password_hash to async def and updated all call sites across backend/auth.py and backend/main.py to await them.

🎯 Why

bcrypt hashing and verification are computationally expensive (by design). When executed directly inside a FastAPI async def endpoint (like /api/auth/login), these synchronous, CPU-bound operations block the asyncio event loop for the duration of the hashing process. This prevents the server from handling any other concurrent requests, severely degrading throughput and increasing latency during authentication bursts. Offloading them to a threadpool ensures the event loop remains responsive.

📊 Impact

Prevents event loop blocking during password verification and hashing. Expected to significantly improve concurrent request throughput (QPS) and reduce latency jitter for all non-authentication endpoints during periods of high login/registration activity.

🔬 Measurement

Run pytest to ensure authentication logic remains functional. Profile event loop responsiveness (e.g., using asyncio debug mode or load testing tools like locust/wrk) while simulating multiple concurrent login requests. You should observe that non-auth requests are no longer queued/delayed behind auth requests.

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PR created automatically by Jules for task 4530374919660479394 started by @benpiper

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