🛡️ Sentinel: [MEDIUM] Fix swallowed exceptions in custom global error handler

[n24q02m]

🚨 Severity: MEDIUM
💡 Vulnerability: The FastAPI application used a custom global exception handler to apply security headers to 500 error responses, but this overrode Starlette's built-in ServerErrorMiddleware logging. It returned a plain 500 response but silently swallowed the traceback on the server-side, making debugging impossible and masking potential exploitation attempts.
🎯 Impact: Prevents operators from diagnosing internal server errors or identifying malicious activity causing repeated 500 responses.
🔧 Fix: Added log.error("Unhandled server error", exc_info=_exc) to the global exception handler to explicitly log the exception before returning the safe generic response.
✅ Verification: Verified by inspecting the src/tacet/serve/server.py implementation and ensuring the full test suite passes. Also ran the specific test_mvp.py endpoints.

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PR created automatically by Jules for task 12151315433162222503 started by @n24q02m

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