65b7daff0b
Address PR-agent review findings: - resolveVoice fast path: replace ambiguous boolean comparison (voiceProvider === "stepfun") === serverStepfun with explicit per-provider equality checks. Prevents an undefined or unknown provider from matching the non-stepfun (xiaomi) branch by accident. - /api/beat-audio route: reject requests whose voice.provider is present but not in the VALID_TTS_PROVIDERS whitelist (e.g. "azure"). Previously such a request would pass validation when fallback fields were also present, and resolveVoice might use the invalid voice directly instead of falling back to reprovision — producing a silent beat instead of a voiced one.