Add IBM Watson Text to Speech as an enterprise cloud provider option #59

IBM Watson Text to Speech is useful for enterprise-oriented workflows, custom/branded voices, and organizations already invested in IBM Cloud or Watson services. It should be considered after the provider abstraction and higher-priority providers are in place.

Impact:

• Enterprise users who rely on IBM Cloud cannot use their preferred TTS backend.
• The app has no path for Watson-specific voice customization or branded voice workflows.

Suggested fix:

• Add an IBM Watson TTS provider implementation behind the provider abstraction.
• Add settings for IBM API key, service URL/region, voice, output format, and any supported SSML controls.
• Normalize Watson responses into the same audio result model used by other providers.
• Surface provider-specific errors and unsupported capability differences clearly.
• Add mocked provider tests and setup documentation.