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.Api/Controllers/HealthController.cs

@@ -0,0 +1,29 @@
+using Microsoft.AspNetCore.Mvc;
+
+namespace TuneRipper.Api.Controllers;
+
+[ApiController]
+[Route("api/[controller]")]
+public class HealthController : ControllerBase
+{
+    [HttpGet]
+    public IActionResult Get()
+    {
+        var response = new
+        {
+            status = "healthy",
+            timestamp = DateTime.UtcNow,
+            version = "1.0.0",
+            service = "TuneRipper.Api",
+            environment = Environment.GetEnvironmentVariable("ASPNETCORE_ENVIRONMENT") ?? "Production"
+        };
+
+        return Ok(response);
+    }
+
+    [HttpGet("ping")]
+    public IActionResult Ping()
+    {
+        return Ok(new { message = "pong", timestamp = DateTime.UtcNow });
+    }
+}