Refactor downloads to use job-specific subdirectories

Organize downloaded files into per-job subdirectories under "downloads" instead of a flat structure. Update DownloadController and VideoService to read, write, and delete files within these job-specific directories. Add helper methods for directory management and improve logging. Update download/delete endpoints and result URLs to reflect the new structure. Clean up empty job directories after file deletion.

TuneRipper.Api/Services/ISessionService.cs

@@ -0,0 +1,10 @@
+namespace TuneRipper.Api.Services;
+
+public interface ISessionService
+{
+    string CreateSession();
+    string GetOrCreateSession(string? sessionId);
+    bool IsValidSession(string sessionId);
+    void CleanupExpiredSessions();
+    string GetSessionDownloadsPath(string sessionId);
+}