feat: pre-build playlist cache and make matching interval configurable

- Pre-build playlist items cache during track matching for instant serving
- Add PreBuildPlaylistItemsCacheAsync() to SpotifyTrackMatchingService
- Combines local Jellyfin tracks + external matched tracks in correct Spotify order
- Saves to both Redis and file cache for persistence across restarts
- Change matching interval from hardcoded 30 minutes to configurable (default: 24 hours)
- Add SPOTIFY_IMPORT_MATCHING_INTERVAL_HOURS environment variable
- Set to 0 to only run once on startup (manual trigger still works)
- Add endpoint usage files to .gitignore
- Update documentation in README and .env.example

Rationale: Spotify playlists like Discover Weekly update once per week,
so running every 24 hours is more than sufficient. Pre-building the cache
eliminates slow 'on the fly' playlist building.

All 225 tests pass.

allstarr/Models/Settings/SpotifyImportSettings.cs

@@ -80,6 +80,15 @@ public class SpotifyImportSettings
     /// </summary>
     public int SyncWindowHours { get; set; } = 2;
     
+    /// <summary>
+    /// How often to run track matching in hours.
+    /// Spotify playlists like Discover Weekly update once per week, Release Radar updates weekly.
+    /// Most playlists don't change frequently, so running every 24 hours is reasonable.
+    /// Set to 0 to only run once on startup (manual trigger via admin UI still works).
+    /// Default: 24 hours
+    /// </summary>
+    public int MatchingIntervalHours { get; set; } = 24;
+    
     /// <summary>
     /// Combined playlist configuration as JSON array.
     /// Format: [["Name","Id","first|last"],...]