Extract storage key building into a shared StorageKey helper

Application/Shared/StorageKey.cs

@@ -0,0 +1,12 @@
+namespace Musify.Application.Shared
+{
+    public static class StorageKey
+    {
+        public static string Combine(params string[] segments)
+        {
+            return string.Join('/', segments
+                .Where(static s => !string.IsNullOrWhiteSpace(s))
+                .Select(static s => s.Trim().Trim('/', '\\')));
+        }
+    }
+}