fix(backend): eagerly resolve PolicyRunner at startup for fail-fast on missing ONNX

Backend/Program.cs

@@ -17,6 +17,15 @@ builder.Services.AddSingleton<PolicyRunner>(sp =>
 builder.Services.AddLogging();
 
 var app = builder.Build();
+
+// Fail-fast: eagerly resolve PolicyRunner so a missing ONNX file surfaces at
+// startup rather than on the first WebSocket connect.
+var startupLogger = app.Services.GetRequiredService<ILoggerFactory>().CreateLogger("Startup");
+var policy = app.Services.GetRequiredService<PolicyRunner>();
+var modelPath = PathResolver.Resolve(
+    app.Services.GetRequiredService<Microsoft.Extensions.Options.IOptions<LanderConfig>>().Value.ModelPath);
+startupLogger.LogInformation("Loaded PPO policy from {ModelPath}", modelPath);
+
 app.UseWebSockets();
 
 // -------- Health --------