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>

GerbilManager.AppHost/AppHost.cs

@@ -1,3 +1,12 @@
-var builder = DistributedApplication.CreateBuilder(args);
-
-builder.Build().Run();
+var builder = DistributedApplication.CreateBuilder(args);
+
+var postgres = builder.AddPostgres("postgres")
+    .WithDataVolume();
+
+var gerbilDb = postgres.AddDatabase("gerbilmanager");
+
+builder.AddProject<Projects.GerbilManagerWebAPI>("webapi")
+    .WithReference(gerbilDb)
+    .WaitFor(gerbilDb);
+
+builder.Build().Run();