⚡ Bolt: Offload CPU-bound bcrypt operations to threadpool

[benpiper]

💡 What: Modified verify_password and get_password_hash in backend/auth.py to be async and offloaded the underlying bcrypt operations to Starlette's threadpool using starlette.concurrency.run_in_threadpool. Updated callers in backend/main.py and backend/auth.py to await these functions.

🎯 Why: bcrypt.checkpw and bcrypt.hashpw are intentionally slow, CPU-bound operations. When executed directly within a FastAPI async def endpoint (such as /api/auth/login), they block the event loop entirely. This means while one user is logging in, no other async requests (like video processing status checks or media serving) can be handled, causing severe performance degradation under load.

📊 Impact: Prevents the FastAPI event loop from stalling during login and registration. This dramatically improves the concurrency and responsiveness of the application under multi-user load, allowing other endpoints to continue serving requests while password hashes are computed on background threads.

🔬 Measurement: Tested using pytest to ensure authentication logic remains completely functional. Concurrency can be verified by running a load test against /api/auth/login and simultaneously hitting another fast endpoint like /health—the fast endpoint will no longer stall waiting for the login requests to finish.

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PR created automatically by Jules for task 14395000185252322556 started by @benpiper

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