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)

.env.example

@@ -116,18 +116,6 @@ CACHE_DURATION_HOURS=1
 # Enable Spotify playlist injection (optional, default: false)
 SPOTIFY_IMPORT_ENABLED=false
 
-# Sync schedule: When does the Spotify Import plugin run?
-# Used for the sync window check to avoid fetching too frequently
-# Set these to match your plugin's sync schedule in Jellyfin
-# Example: If plugin runs daily at 4:15 PM, set HOUR=16 and MINUTE=15
-SPOTIFY_IMPORT_SYNC_START_HOUR=16
-SPOTIFY_IMPORT_SYNC_START_MINUTE=15
-
-# Sync window: How long to search for missing tracks files (in hours)
-# The fetcher will check within this window after the sync start time
-# Example: If plugin runs at 4:15 PM and window is 2 hours, checks from 4:15 PM to 6:15 PM
-SPOTIFY_IMPORT_SYNC_WINDOW_HOURS=2
-
 # Matching interval: How often to run track matching (in hours)
 # Spotify playlists like Discover Weekly update once per week, Release Radar updates weekly
 # Set to 0 to only run once on startup (manual trigger via admin UI still works)