🛡️ Sentinel: [HIGH] Fix Event Loop Blocking DoS in Auth

[benpiper]

Severity: HIGH
Vulnerability: CPU-bound operations (like bcrypt hashing) were running directly in async route handlers.
Impact: Because bcrypt operations are computationally intensive, doing them directly inside an async def function blocks the FastAPI asyncio event loop. This leads to a severe performance degradation and opens the application up to Denial of Service (DoS) attacks, as it stops the server from handling concurrent requests during the hashing process.
Fix: Offloaded the bcrypt operations (hashpw and checkpw) to a separate thread pool using starlette.concurrency.run_in_threadpool and wrapped them in new async functions verify_password and get_password_hash. Updated usages in backend/auth.py and backend/main.py.
Verification: Verified changes by running the backend test suite (pytest) and checking linting (ruff), making sure no regressions were introduced while the blocking DoS potential was eliminated.

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PR created automatically by Jules for task 1001470159459866222 started by @benpiper

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