Initial implementation of TuneRipper API backend

- Add Health, Video, and Download controllers with endpoints for health checks, video details, processing, status, results, and simulated downloads
- Implement IVideoService for YouTube URL validation and mock video info
- Add API models for responses and job status tracking
- Configure DI, CORS (BlazorPolicy), OpenAPI, and root metadata endpoint in Program.cs
- Add README with usage, troubleshooting, and development notes
- Minor project file cleanup and encoding fix

TuneRipper.Shared/Models/ApiModels.cs

@@ -0,0 +1,28 @@
+namespace TuneRipper.Shared.Models;
+
+public class ApiResponse<T>
+{
+    public bool Success { get; set; }
+    public string? Message { get; set; }
+    public T? Data { get; set; }
+    public List<string> Errors { get; set; } = new();
+}
+
+public class JobStatusResponse
+{
+    public string? JobId { get; set; }
+    public string? Status { get; set; }
+    public int PercentageComplete { get; set; }
+    public string? Message { get; set; }
+    public DateTime CreatedAt { get; set; }
+    public DateTime? CompletedAt { get; set; }
+    public ProcessVideoResponse? Result { get; set; }
+}
+
+public class VideoInfo
+{
+    public string? Url { get; set; }
+    public string? VideoId { get; set; }
+    public bool IsValid { get; set; }
+    public string? ErrorMessage { get; set; }
+}