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/TuneRipper.Api.csproj

@@ -1,4 +1,4 @@
-<Project Sdk="Microsoft.NET.Sdk.Web">
+<Project Sdk="Microsoft.NET.Sdk.Web">
 
   <PropertyGroup>
     <TargetFramework>net10.0</TargetFramework>
@@ -14,8 +14,4 @@
     <ProjectReference Include="..\TuneRipper.Shared\TuneRipper.Shared.csproj" />
   </ItemGroup>
 
-  <ItemGroup>
-    <Folder Include="Controllers\" />
-  </ItemGroup>
-
 </Project>