🛡️ Sentinel: [CRITICAL] Fix user enumeration timing attack

[luisdlopera]

🚨 Severity: CRITICAL
💡 Vulnerability: A timing attack existed in the login.use-case.ts because bcryptjs.compare was given an invalid hash format when an unknown email was provided. This caused bcryptjs.compare to fail instantly instead of doing the work, creating a noticeable time difference between existing and non-existing users.
🎯 Impact: Attackers could enumerate user emails and figure out who has an account in the system by measuring the response time.
🔧 Fix: Replaced the invalid hash string ($2a$12$dummyhash...) with a precomputed valid bcrypt hash string ($2b$12$...) so the computation takes the expected amount of time.
✅ Verification: Ran the backend test suite successfully (cd back && pnpm test) and validated linting passed.
📓 Journaled learning in .jules/sentinel.md

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PR created automatically by Jules for task 7225344166529140090 started by @luisdlopera

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