🛡️ Sentinel: [HIGH] Add rate limiting to login endpoint

[benpiper]

🚨 Severity: HIGH
💡 Vulnerability: Brute-force risk on login endpoint due to lack of specific rate-limiting. The global rate limit is too permissive for sensitive auth routes.
🎯 Impact: Attackers could launch high-volume password guessing attacks against user accounts without being restricted.
🔧 Fix: Introduced _login_rate_limiter in backend/main.py using a separate PerIPRateLimiter configured specifically for the login route, defaulting to 5 calls per 300 seconds. Added a check within the /api/auth/login endpoint that raises a 429 error if the limit is exceeded.
✅ Verification:

1. Confirmed the modification locally by reviewing backend/main.py.
2. Cleaned up scratchpad script.
3. Successfully ran formatting, linting (uv run ruff check .), and the backend test suite (uv run pytest tests/).
4. Logged learning to .jules/sentinel.md.

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PR created automatically by Jules for task 3901587921100545985 started by @benpiper

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