Summary
Major update: background removal, UI improvements, model fixes, and CI stability.
Changes

• Added RemoveBackgroundJob with rembg CLI for automatic background removal
• Created Image and DesignEditSession models; updated schema comments
• Added brand.css and regulated form partial for new/edit designs
• Added dark mode support and image preview scaling
• Enabled PostgreSQL extensions; switched dev DB to SQLite
• Removed stale migrations (verified, trust columns from users)
• Cleaned up model validations and associations
• Fixed CI test suite with updated fixtures
• Removed node_modules from git; added to .gitignore
• Removed unused editor and dashboard views
• Updated Gemfile/Gemfile.lock dependencies