Generate Batch Voices ≤500 total chars

Batch text-to-speech: synthesize up to 20 separate text items in a single call, standard voices only, up to 500 total characters, returns JSON with base64 WAV audio plus duration and sample rate per item. Use it to: voice a list of short UI strings at once, generate audio for multiple notification templates, batch-produce voice lines for a menu or form, synthesize several short replies in one request. USDC on Base via x402.