feat(api): extract pure API clients for image generation and TTS

- Add bin/api/ package with generate_image() and synthesize_tts()
- Refactor bin/commands/image.py to call bin.api.image.generate_image
- Refactor bin/commands/voice.py to call bin.api.voice.synthesize_tts
- API layer is free of Click CLI dependencies, usable by external scripts

bin/api/__init__.py

@@ -0,0 +1,6 @@
+"""Pure API clients for external services (no CLI deps)."""
+
+from .image import generate_image
+from .voice import synthesize_tts
+
+__all__ = ["generate_image", "synthesize_tts"]