feat: spotify playlist injection

- Add SpotifyImportSettings configuration model
- Create SpotifyMissingTracksFetcher background service
- Inject virtual Spotify playlists into search results
- Auto-match tracks from external providers
- Update README with feature documentation
- Configure sync window and playlist settings

allstarr/appsettings.Development.json

@@ -4,5 +4,25 @@
       "Default": "Information",
       "Microsoft.AspNetCore": "Warning"
     }
+  },
+  "SpotifyImport": {
+    "Enabled": false,
+    "JellyfinUrl": "http://localhost:8096",
+    "ApiKey": "",
+    "SyncStartHour": 16,
+    "SyncStartMinute": 15,
+    "SyncWindowHours": 2,
+    "Playlists": [
+      {
+        "Name": "Release Radar",
+        "SpotifyName": "Release Radar",
+        "Enabled": true
+      },
+      {
+        "Name": "Discover Weekly",
+        "SpotifyName": "Discover Weekly",
+        "Enabled": true
+      }
+    ]
   }
 }