ARCH-2: wire PostgreSQL through Aspire

- AppHost: AddPostgres(postgres).WithDataVolume() + AddDatabase(gerbilmanager),
  WebAPI project resource WithReference/WaitFor on the db (Aspire-generated credentials)
- WebAPI: AddNpgsqlDbContext<ApplicationContext>(gerbilmanager) replaces manual
  AddDbContext/UseNpgsql + appsettings connection string

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

GerbilManagerWebAPI/Program.cs

@@ -11,7 +11,7 @@ builder.Services.AddControllers();
 // Learn more about configuring Swagger/OpenAPI at https://aka.ms/aspnetcore/swashbuckle
 builder.Services.AddEndpointsApiExplorer();
 builder.Services.AddSwaggerGen();
-builder.Services.AddDbContext<ApplicationContext>(opts => opts.UseNpgsql(builder.Configuration.GetConnectionString("gerbilmanager")));
+builder.AddNpgsqlDbContext<ApplicationContext>("gerbilmanager");
 builder.Services.AddScoped<UnitOfWork>();
 
 var app = builder.Build();