🛡️ Sentinel: [MEDIUM] Fix swallowed exception tracebacks in custom FastAPI handler

[n24q02m]

🚨 Severity: MEDIUM
💡 Vulnerability: Swallowed server-side exception tracebacks. Overriding the default global Exception handler in FastAPI bypasses Starlette's default trace logging.
🎯 Impact: Server-side errors and potential exploitation attempts become invisible to operators because unhandled exceptions are silently swallowed, breaking the observability of 500 errors.
🔧 Fix: Added explicit logging.error("Unhandled server exception", exc_info=_exc) in unhandled_exception_handler to securely log traces on the server while still returning a generic plain-text response to clients. Added journal learning to .jules/sentinel.md.
✅ Verification: Tested via the existing test suite (uv run --all-extras pytest) and confirmed the generic 500 response behavior remains unchanged. Validated via ruff check and ruff format.

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PR created automatically by Jules for task 2366684217478521217 started by @n24q02m

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