⚡ Bolt: Offload CPU-bound bcrypt operations to threadpool

[benpiper]

💡 What

Updated backend/auth.py to offload CPU-bound bcrypt.hashpw and bcrypt.checkpw operations to the background threadpool using starlette.concurrency.run_in_threadpool. Made verify_password and get_password_hash asynchronous, and updated all call sites in backend/main.py and backend/auth.py to await them. Added an entry to .jules/bolt.md recording this pattern.

🎯 Why

In FastAPI/Starlette, asynchronous routes and event loops are designed for I/O bounds. When CPU-bound tasks (like bcrypt, which is intentionally slow by design, often taking hundreds of milliseconds) are run directly on the event loop, they block it entirely. This means during a login or registration request, all other concurrent requests to the API stall until the hash completes. Moving these to a threadpool frees up the main event loop to handle other requests concurrently.

📊 Impact

Increases API throughput and concurrency under load. Background tasks and other API endpoints will no longer be stalled for ~100-300ms during every login/registration attempt.

🔬 Measurement

Tests were run via cd backend && uv run pytest tests/ to confirm functionality remains unchanged. Manual timing scripts (like the one implemented during exploration) show background task iterations jumping from ~44 to ~140 during 3 concurrent login simulations, showing the event loop remains unblocked.

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PR created automatically by Jules for task 14360453699703027549 started by @benpiper

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