Standardize .env.example files with complete variables and empty values

Changes:
- Reviewed all docker-compose files to identify required variables
- Updated all .env.example files to include ONLY variables used in their respective compose files
- Changed all placeholder values to empty strings for cleaner configuration
- Fixed variable name inconsistency: DOWNLOADS_DIR -> DOWNLOAD_DIR (to match main .env)
- Removed unnecessary variables that aren't used by the compose files

All .env.example files now:
- Contain only the variables actually used in the stack
- Have empty values instead of example placeholders
- Match the variable names used in docker-compose files

This ensures clean, minimal .env files for each stack without any extraneous variables or confusing placeholder values.

docker-compose/vpn/docker-compose.yml

@@ -71,7 +71,7 @@ services:
     network_mode: 'service:gluetun'  # Routes through VPN in same compose file
     volumes:
       - ./qbittorrent/config:/config
-      - ${DOWNLOADS_DIR}:/downloads
+      - ${DOWNLOAD_DIR}:/downloads
     environment:
       - PUID=${PUID}
       - PGID=${PGID}