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.

.env.example

@@ -126,6 +126,13 @@ SPOTIFY_IMPORT_SYNC_START_MINUTE=15
 # Example: If plugin runs at 4:15 PM and window is 2 hours, checks from 4:00 PM to 6:00 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
+# Most playlists don't change frequently, so running once per day is reasonable
+# Set to 0 to only run once on startup (manual trigger via admin UI still works)
+# Default: 24 hours
+SPOTIFY_IMPORT_MATCHING_INTERVAL_HOURS=24
+
 # Playlists configuration (JSON ARRAY FORMAT - managed by web UI)
 # Format: [["PlaylistName","SpotifyPlaylistId","first|last"],...]
 # - PlaylistName: Name as it appears in Jellyfin