feat: add Jellyfin config backup to Restic

Added jellyfin-backup plan to Backrest configuration:
- Backs up /volumes/jelly_config daily at 9 AM
- Retention: 7 daily, 4 weekly, 6 monthly, 2 yearly
- Added jelly_config volume mount to restic/compose.yaml
- Updated documentation in CLAUDE.md

This ensures Jellyfin configuration, library metadata, and user
settings are backed up to HiDrive.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

restic/compose.yaml

@@ -30,6 +30,7 @@ services:
       - backup_letsencrypt_data:/volumes/letsencrypt_data:ro
       - backup_vaultwarden_data:/volumes/vaultwarden_data:ro
       - backup_joplin_data:/volumes/joplin_data:ro
+      - backup_jelly_config:/volumes/jelly_config:ro
 
     environment:
       TZ: ${TIMEZONE:-Europe/Berlin}
@@ -113,6 +114,10 @@ volumes:
     name: joplin_data
     external: true
 
+  backup_jelly_config:
+    name: jelly_config
+    external: true
+
 networks:
   compose_network:
     name: ${NETWORK_NAME}