refactor: remove sync window logic from Spotify Import

- Simplified SpotifyMissingTracksFetcher to remove complex sync window timing
- Now fetches on startup if cache missing, then checks every 5 minutes for stale cache (>24h)
- Removed SYNC_START_HOUR, SYNC_START_MINUTE, SYNC_WINDOW_HOURS from config
- Updated README and .env.example to reflect simpler configuration
- Sync window was only relevant for legacy Jellyfin plugin scraping method
- When using sp_dc cookie method (recommended), this service is dormant anyway
- Deleted MIGRATION.md (local-only file, not for repo)

allstarr/Controllers/AdminController.cs

@@ -166,8 +166,7 @@ public class AdminController : ControllerBase
             spotifyImport = new
             {
                 enabled = _spotifyImportSettings.Enabled,
-                syncTime = $"{_spotifyImportSettings.SyncStartHour:D2}:{_spotifyImportSettings.SyncStartMinute:D2}",
-                syncWindowHours = _spotifyImportSettings.SyncWindowHours,
+                matchingIntervalHours = _spotifyImportSettings.MatchingIntervalHours,
                 playlistCount = _spotifyImportSettings.Playlists.Count
             },
             deezer = new
@@ -1392,9 +1391,7 @@ public class AdminController : ControllerBase
             spotifyImport = new
             {
                 enabled = _spotifyImportSettings.Enabled,
-                syncStartHour = _spotifyImportSettings.SyncStartHour,
-                syncStartMinute = _spotifyImportSettings.SyncStartMinute,
-                syncWindowHours = _spotifyImportSettings.SyncWindowHours,
+                matchingIntervalHours = _spotifyImportSettings.MatchingIntervalHours,
                 playlists = _spotifyImportSettings.Playlists.Select(p => new
                 {
                     name = p.Name,