🛡️ Sentinel: [MEDIUM] Fix exception handling information leak in admin login

[shadowcoder8]

🚨 Severity: MEDIUM
💡 Vulnerability: The admin_login route was explicitly catching all exceptions and returning their internal str(e) representation in a 401 response payload.
🎯 Impact: This violated the "fail securely" principle and allowed potential Information Exposure (CWE-209) to attackers if internal systems (like DB or config) threw unexpected errors.
🔧 Fix: Explicitly caught intended HTTPExceptions to re-raise them correctly, while catching general Exceptions separately to return a generic 500 "Internal server error" while maintaining logging.
✅ Verification: An ad-hoc test was created, verified, and deleted, passing fastapi.testclient.TestClient responses to confirm generic exceptions no longer leak data, and pytest was run to ensure no regressions. The Sentinel journal was also updated to record this learning.

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PR created automatically by Jules for task 3329980495604193158 started by @shadowcoder8

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