refactor: reuse Jellyfin settings for Spotify feature

- Remove duplicate JellyfinUrl and ApiKey from SpotifyImportSettings
- Use existing JELLYFIN_URL and JELLYFIN_API_KEY settings
- Simplify configuration - no duplicate settings needed
- Update documentation and .env.example

allstarr/appsettings.Development.json

@@ -7,8 +7,6 @@
   },
   "SpotifyImport": {
     "Enabled": false,
-    "JellyfinUrl": "http://localhost:8096",
-    "ApiKey": "",
     "SyncStartHour": 16,
     "SyncStartMinute": 15,
     "SyncWindowHours": 2,