feat: implement per-playlist cron scheduling with persistent cache

- Added Cronos package for cron expression parsing
- Each playlist now has independent cron schedule (default: 0 8 * * 1)
- Cache persists until next cron run, not just cache duration
- Prevents excess Spotify API calls - only refreshes on cron trigger
- Manual refresh still allowed with 5-minute cooldown
- Added 429 rate limit handling for user playlist fetching
- Added crontab.guru link to UI for easy schedule building
- Both SpotifyPlaylistFetcher and SpotifyTrackMatchingService use cron
- Automatic matching only runs when cron schedule triggers

allstarr/Controllers/AdminController.cs

@@ -1966,6 +1966,12 @@ public class AdminController : ControllerBase
                 var response = await _jellyfinHttpClient.SendAsync(request);
                 if (!response.IsSuccessStatusCode)
                 {
+                    if (response.StatusCode == System.Net.HttpStatusCode.TooManyRequests)
+                    {
+                        _logger.LogWarning("Spotify rate limit hit (429) when fetching playlists");
+                        return StatusCode(429, new { error = "Spotify rate limit exceeded. Please wait a moment and try again." });
+                    }
+                    
                     _logger.LogWarning("Failed to fetch Spotify playlists: {StatusCode}", response.StatusCode);
                     break;
                 }