Add model_options to SmallestAITTSConfiguration's voice field so the UI
renders the correct voice list per model — 15 standard voices for
lightning_v3.1 and 6 premium voices (meher, rhea, aviraj, cressida,
willow, maverick) for lightning_v3.1_pro. All 21 voice IDs verified
against the Smallest AI API. The frontend's existing model_options
machinery already handles dropdown filtering and auto-reset on model
change, so no UI changes are needed.