perf(backend): cache YouTube searches and shrink the thumbnail URLs

Repeated searches and song lookups now reuse a single-flight memory cache, so
concurrent or debounced queries hit YouTube once instead of on every keystroke.
Thumbnails are resolved at a configurable size instead of the largest variant,
which cuts the image payload the browser pulls from Google.

backend/Musify.Infrastructure/Configuration/YouTubeConfiguration.cs

@@ -14,5 +14,14 @@ namespace Musify.Infrastructure.Configuration
 
         [Range(60, 3600)]
         public int SearchContinuationCacheSeconds { get; set; } = 600;
+
+        [Range(0, 86400)]
+        public int SearchResultsCacheSeconds { get; set; } = 900;
+
+        [Range(0, 604800)]
+        public int SongInfoCacheSeconds { get; set; } = 86400;
+
+        [Range(32, 720)]
+        public int ThumbnailSize { get; set; } = 240;
     }
 }