Add DALL-E image generator adapter

[adelaidasofia]

The dalle adapter is stubbed in image_generators/dalle.py but not implemented. Contributions welcome.

What's needed:

• Implement DalleAdapter.generate(spec) using the OpenAI Images API
• Honor api_key, model (default dall-e-3), size, and output_dir from config.json under image_generator.dalle
• Build the prompt from the pillar + quote, generate, download to spec["output_path"]
• Follow the ImageGenerator ABC in image_generators/base.py

Spec the adapter receives:

{
  "pillar": "P1" | "P2" | "P3",
  "quote": str,
  "handle": str,
  "figure_path": str | None,
  "output_path": str,
}

See image_generators/pillow_local.py for a complete reference implementation and image_generators/canva.py for an API-driven example.