Adds 'speech-sdk' as a fifth built-in TTS provider backed by @speech-sdk/core
(Apache 2.0). Models use the provider/model format; requests go from the
server directly to the provider's API with the user's own key. Existing
providers, defaults, and config are untouched.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
