🛡️ Sentinel: [HIGH] Fix Event Loop Blocking (DoS)

[benpiper]

🚨 Severity: HIGH

💡 Vulnerability:
Synchronous blocking calls (time.sleep and CPU-bound bcrypt operations) inside async def routes block the FastAPI event loop, leading to Denial of Service for all other concurrent requests.

🎯 Impact:
If an attacker repeatedly triggers password hashing (e.g., by spamming login or registration endpoints) or hits the get_frame_image polling loop while it sleeps, it blocks the main event loop, severely degrading the performance and availability of the entire API for all other users.

🔧 Fix:

1. Offloaded the CPU-bound bcrypt operations in backend/auth.py to a background thread pool using starlette.concurrency.run_in_threadpool.
2. Updated all caller routes in backend/main.py and backend/auth.py to await the new async hashing functions.
3. Replaced the blocking time.sleep(1) in get_frame_image with await asyncio.sleep(1) to yield control back to the event loop.
4. Added an entry to .jules/sentinel.md detailing the learning.

✅ Verification:

1. Ran cd backend && uv run pytest to ensure all existing backend tests pass.
2. Ran cd backend && uv run ruff check . to verify no linting regressions.

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PR created automatically by Jules for task 7757078334069749647 started by @benpiper

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