Add Midjourney image generator adapter

[adelaidasofia]

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

What's needed:

• Implement MidjourneyAdapter.generate(spec) using the Midjourney API (or an unofficial wrapper)
• Honor api_key, aspect_ratio, and output_dir from config.json under image_generator.midjourney
• Poll for completion, download the final image, save 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.

[ericjoye]

It looks like the midjourney adapter needs a full implementation of its generate method, including API interaction, polling, and file saving. I'd approach this by integrating with the official Midjourney API, handling the api_key, aspect_ratio, and output_dir configurations as specified. I've previously built a similar image generation adapter for DALL-E that manages asynchronous job polling and local file storage. If this is still open, I'd be happy to submit a PR for a bounty: https://github.com/ericjoye