fix(config): register parakeet-cpp as a transcript backend (#9718) (#10106)

parakeet-cpp was added in #10084 but not registered in
BackendCapabilities, so GuessUsecases only allowed "whisper" for
FLAG_TRANSCRIPT and the UI could not classify parakeet-cpp models as
speech-to-text. The result was that parakeet models appeared only in
the LLM selector in the speech-to-speech pipeline, making them
unusable for transcription through the UI.

Closes #9718

Assisted-by: Claude Sonnet 4.6 <noreply@anthropic.com>