Add Gradio API mode for programmatic access

[dreamlessx]

Description

Add an API endpoint mode to the Gradio demo that allows programmatic access without the web UI, enabling integration with other tools and pipelines.

Current Behavior

The Gradio app (scripts/app.py) only serves the interactive web interface. Users who want to integrate LandmarkDiff into their pipelines must call the Python API directly.

Proposed Changes

1. Add a /api/predict endpoint that accepts JSON with base64-encoded images
2. Return JSON with the result image, landmarks, and metadata
3. Add a --api-only flag to launch without the Gradio UI (lighter weight)
4. Document the API schema with example curl commands

Acceptance Criteria

• POST /api/predict accepts {"image": "<base64>", "procedure": "rhinoplasty", "intensity": 65}
• Response includes {"result": "<base64>", "landmarks": [...], "procedure": "..."}
• --api-only flag starts a lightweight FastAPI/Flask server
• README documents the API with curl examples

Labels

enhancement