🛡️ Sentinel: [CRITICAL] Fix user enumeration timing attack in login

[luisdlopera]

🚨 Severity: CRITICAL
💡 Vulnerability: User Enumeration via Timing Attack. The dummy bcrypt hash used to normalize response times when a user is not found was malformed. bcryptjs rejects improperly formatted strings almost instantly without performing the expensive hashing computation. This resulted in the API responding significantly faster for non-existent users than for valid users.
🎯 Impact: Attackers could determine which email addresses exist in the system by measuring the response time of the login endpoint.
🔧 Fix: Replaced the malformed hash with a valid, pre-computed dummy hash using the exact same cost factor (10) as legitimate password hashes. This ensures that the response time is consistent regardless of whether the user exists. Also added an entry to the .jules/sentinel.md journal.
✅ Verification: Verified by checking that bcryptjs.compare executes the CPU-intensive work and response times are balanced. Also ran tests and linter successfully.

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PR created automatically by Jules for task 15165739727171900646 started by @luisdlopera

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