Pre-fill list section on product pick; tighten backend warnings

- Adds GET /api/products/{kind}/{id}/section?storeId=... exposing the
  per-store memory the list page mirrors when a product is picked, so the
  section dropdown reflects what the backend would auto-assign on POST.
- Treats backend warnings as errors via Directory.Build.props; fixes the
  surfaced warnings (obsolete PostgreSqlBuilder ctor, nullable string[]
  in IsEquivalentTo, redundant nullable flow).
- Annotates wire-exposed enums (ProductKind, UnitKind, UnitCategory,
  UnitCategoryFlags) with JsonStringEnumConverter so they round-trip as
  strings regardless of caller options. Unblocks the integration tests
  that deserialize DTOs via GetFromJsonAsync without the global converter.

src/backend/YesChef.Api.IntegrationTests/Infrastructure/PostgresFixture.cs

@@ -1,4 +1,3 @@
-using Microsoft.EntityFrameworkCore;
 using Npgsql;
 using Testcontainers.PostgreSql;
 using TUnit.Core.Interfaces;
@@ -16,8 +15,7 @@ public sealed class PostgresFixture : IAsyncInitializer, IAsyncDisposable
 {
     private const string TemplateDbName = "yeschef_template";
 
-    private readonly PostgreSqlContainer _container = new PostgreSqlBuilder()
-        .WithImage("postgres:17")
+    private readonly PostgreSqlContainer _container = new PostgreSqlBuilder("postgres:17")
         .WithDatabase("postgres")
         .WithUsername("postgres")
         .WithPassword("postgres")