Refactor WebUI backend structure

[LumePart]

This PR focuses on restructuring the WebUI backend and improving the overall project layout. No major logic changes are expected

Changes

• Moved files into dedicated sub-packages to improve navigation and keep related functionality grouped together:

  • app – shared backend configuration and application state
  • auth - authentication, middleware package
  • defs – shared static configuration structs and variables
  • jobs – core cron job logic
  • playlist – custom playlist import parsing and handling
  • run – Server-Sent Events (SSE) and manual run functionality
  • settings – logic for reading, updating, and saving the .env configuration file

The goal of this restructuring is to make the backend easier to navigate, maintain, and extend as the project grows.

This is still WiP