🛡️ Sentinel: Limit transcription history to prevent DoS

[google-labs-jules]

Implemented a limit on the transcription history to prevent potential Denial of Service (DoS) through memory exhaustion. Also fixed a race condition in the transcription update logic found during testing. Verified with a new security test case.

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PR created automatically by Jules for task 14523812342321113942 started by @tblakex01

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