Add debug logging for received image data

Fix base64 data URI detection: use startsWith instead of comma check

Change custom API path to /custom_api/ to avoid Gradio /api/ conflict

Use FastAPI + mount_gradio_app for custom /api/digitize_base64 endpoint

Add base64 API endpoint for programmatic access

Initial deploy: AutoLineDigitizer Gradio API for HuggingFace Spaces