designs to polymorphic model full

[goodeats]

The scope of this PR got bigger bc:

• made breaking changes on purpose before finishing feature with the idea this wouldn't come back to bite me
• testing out flat /app/models file structure in style of flat routes in order to
• testing out using a /dev/ file with docs and template files to use as references in Cursor prompts

Making a checklist to ensure completion:

• complete template files
• complete model files for all existing models
• all designs to polymorphic model with type-specific
• flesh out list and create sub-tasks so I can play with AI agents to code the rest of this for me (or reduce a great amount of it)

This all kind of blew up when I moved in with my gf and summer plans took over, sorry. FWIW this is still a personal project and I would never put so much on one PR in a professional project with actual users and collaborators.