ARCH-2: add Aspire AppHost + ServiceDefaults (Aspire 13.4.2)

- New projects GerbilManager.AppHost / GerbilManager.ServiceDefaults (templates)
- Added both to GerbilManager.slnx
- WebAPI references ServiceDefaults; AddServiceDefaults + MapDefaultEndpoints wired

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

GerbilManagerWebAPI/Program.cs

@@ -3,6 +3,8 @@ using Microsoft.EntityFrameworkCore;
 
 var builder = WebApplication.CreateBuilder(args);
 
+builder.AddServiceDefaults();
+
 // Add services to the container.
 
 builder.Services.AddControllers();
@@ -14,6 +16,8 @@ builder.Services.AddScoped<UnitOfWork>();
 
 var app = builder.Build();
 
+app.MapDefaultEndpoints();
+
 // Configure the HTTP request pipeline.
 if (app.Environment.IsDevelopment())
 {