Add fallback-tier TTS provider using Piper via OpenedAI Speech for
ultra-lightweight CPU-only synthesis. Maps 6 standard OpenAI voice
names (alloy, echo, fable, onyx, nova, shimmer) to Piper voices.
Update factory to use the new PiperTtsProvider class, replacing the
inline stub. Includes 37 unit tests covering provider identity,
voice mapping, and voice listing.
Fixes#395
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
