feat(backend): scaffold ASP.NET Core Web API + xunit test project

Backend/Backend.csproj

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+<Project Sdk="Microsoft.NET.Sdk.Web">
+
+  <PropertyGroup>
+    <TargetFramework>net8.0</TargetFramework>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.AspNetCore.OpenApi" Version="8.0.29" />
+    <PackageReference Include="Microsoft.ML.OnnxRuntime" Version="1.19.*" />
+    <PackageReference Include="Swashbuckle.AspNetCore" Version="6.6.2" />
+  </ItemGroup>
+
+</Project>

Backend/Backend.http

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+@Backend_HostAddress = http://localhost:5281
+
+GET {{Backend_HostAddress}}/weatherforecast/
+Accept: application/json
+
+###

Backend/Program.cs

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+// Placeholder. Rewritten in Task 6.
+var builder = WebApplication.CreateBuilder(args);
+var app = builder.Build();
+app.MapGet("/", () => "GameCli Backend placeholder");
+app.Run();

Backend/Properties/launchSettings.json

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+{
+  "$schema": "http://json.schemastore.org/launchsettings.json",
+  "iisSettings": {
+    "windowsAuthentication": false,
+    "anonymousAuthentication": true,
+    "iisExpress": {
+      "applicationUrl": "http://localhost:7830",
+      "sslPort": 0
+    }
+  },
+  "profiles": {
+    "http": {
+      "commandName": "Project",
+      "dotnetRunMessages": true,
+      "launchBrowser": true,
+      "launchUrl": "swagger",
+      "applicationUrl": "http://localhost:5281",
+      "environmentVariables": {
+        "ASPNETCORE_ENVIRONMENT": "Development"
+      }
+    },
+    "IIS Express": {
+      "commandName": "IISExpress",
+      "launchBrowser": true,
+      "launchUrl": "swagger",
+      "environmentVariables": {
+        "ASPNETCORE_ENVIRONMENT": "Development"
+      }
+    }
+  }
+}

Backend/appsettings.json

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+{
+  "Logging": {
+    "LogLevel": {
+      "Default": "Information",
+      "Microsoft.AspNetCore": "Warning"
+    }
+  },
+  "AllowedHosts": "*",
+  "Lander": {
+    "CliPath": "publish/GameCli/GameCli",
+    "ModelPath": "models/ppo_lander.onnx"
+  }
+}