Backwards compatible pydantic response models

[akaszynski]

Summary

Adds generated Pydantic field documentation from the pinned Keepa backend Javadocs, improves backend model/API docs, and expands the main docs page with clearer return shapes and richer usage examples.

Highlights

• Documents backend model fields programmatically via the model generator.
• Fixes internal API/model links and enables stricter Sphinx reference checking.
• Adds copy buttons to code blocks.
• Keeps dictionary responses as the default for backwards compatibility while documenting the planned move to typed responses.