fix: resolve "No poster found" errors in batch processing (#73)
- Fix inconsistent authentication in jellyfin_service.py
- Standardize all API calls to use header-based auth (X-Emby-Token)
- Remove duplicate API key URL parameters causing HTTP 400 errors
- Fix library item queries to use ItemId instead of Id
- Add enhanced error logging for better debugging
- Add debug scripts for troubleshooting Jellyfin connectivity
Resolves authentication conflicts that caused poster downloads to fail
on certain Jellyfin configurations while working on others.
Tested with Jellyfin v10.10.7.