Refactor container layout: standardize paths, simplify volumes, and improve playlist/cover handling

[drolex2]

• Directory and volume layout
  • Moved application files into /app.
  • Introduced a dedicated /data mount for app data (logs, config, cover).
  • Simplified volume usage: only a single volume/bind is required for app data, plus explicit mounts for music and playlists.
• Playlist directory handling
  • Clarified that Jellyfin’s playlist directory must be mounted into the JellyJams container for direct file access.
  • Removed the PLAYLIST_FOLDER variable and standardized the container’s playlist directory at /playlists (no need for it to be configurable).
  • Fixed: when artist name has special characters, playlist and cover image were saving to different directories.
• Covers and artwork
  • Build process now copies default cover art into /data/cover/ if not already present, allowing users to customize covers in the host appdata directory.
  • Removed redundant cover art checks in start.sh.
  • Simplified and increased speed of cover art search logic to only scan the mounted music directory (since the container cannot see other host paths).
  • Added Jellyfin API method to get artist image by artist name and try this before searching the artist directory.
  • Added support for .webp, .avif, and .bmp image formats in all image searches.
  • Generated covers are saved as .webp images for better combination of quality and compression
• Configuration and environment variables
  • Added explicit environment variables for all required mounts and reflected these in docker-compose.yml.
  • Added default values for essential variables in docker-compose.yml
  • Removed all named volumes from docker-compose.yml in favor of an appdata bind.
  • Removed the need for a separate docker-compose-unraid.yml; recommend removing it.
• Ports
  • Now using WEB_PORT for internal and external port.
  • This is helpful for anyone using the host network with other services that may use port 5000.
• Logging
  • Removed forced DEBUG logging to respect the user's LOG_LEVEL setting.
• General cleanup
  • Updated other code and instructions (README, SETTINGS) to use new file locations, ports, methods, formats, etc.

[drolex2]

@jonasmore What do you think about this pull request? Will it be merged?

[jonasmore]

Thanks @drolex2 looks good.

The change of the volume mapping simplifies the configuration.

New mapping:
PLAYLIST_DIR_HOST=./jellyfin/data/playlists
MUSIC_DIR_HOST=/host/path/to/music
MUSIC_DIR_CONTAINER=/jellyfin/container/path/to/music