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/GerbilManager.AppHost.csproj

@@ -8,4 +8,12 @@
     <UserSecretsId>b153f50c-bec1-401e-9893-f9df7cff2ebd</UserSecretsId>
   </PropertyGroup>
 
+  <ItemGroup>
+    <PackageReference Include="Aspire.Hosting.PostgreSQL" Version="13.4.2" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\GerbilManagerWebAPI\GerbilManagerWebAPI.csproj" />
+  </ItemGroup>
+
 </Project>