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/Directory.Build.props

@@ -0,0 +1,5 @@
+<Project>
+  <PropertyGroup>
+    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+  </PropertyGroup>
+</Project>

src/backend/YesChef.Api.IntegrationTests/Features/ProductEndpointsTests.cs

@@ -1,5 +1,6 @@
 using System.Net;
 using System.Net.Http.Json;
+using System.Text.Json;
 using YesChef.Api.Entities;
 using YesChef.Api.Features.Products;
 using YesChef.Api.IntegrationTests.Infrastructure;
@@ -343,9 +344,9 @@ public class ProductEndpointsTests : AuthenticatedIntegrationTest
             await db.SaveChangesAsync();
         });
 
-        var results = await Client.GetFromJsonAsync<List<ProductEndpoints.ProductDto>>("/api/products?q=");
+        var results = (await Client.GetFromJsonAsync<List<ProductEndpoints.ProductDto>>("/api/products?q="))!;
 
-        var milk = results!.Single(r => r.Name == "Milk");
+        var milk = results.Single(r => r.Name == "Milk");
         await Assert.That(milk.AllowedUnitCategories).IsEqualTo(UnitCategoryFlags.Volume);
 
         var eggs = results.Single(r => r.Name == "Eggs");
@@ -362,4 +363,132 @@ public class ProductEndpointsTests : AuthenticatedIntegrationTest
         var response = await AnonymousClient.GetAsync("/api/products");
         await Assert.That(response.StatusCode).IsEqualTo(HttpStatusCode.Unauthorized);
     }
+
+    [Test]
+    public async Task Get_global_product_section_returns_remembered_section()
+    {
+        var familyId = await GetFamilyIdAsync();
+        var store = await Data.CreateStoreAsync();
+        var (product, section) = await UseDbAsync(async db =>
+        {
+            var p = new Product { Name = "Bananas-Lookup" };
+            var s = new StoreSection { FamilyId = familyId, StoreId = store.Id, Name = "Produce", SortOrder = 1 };
+            db.Products.Add(p);
+            db.StoreSections.Add(s);
+            await db.SaveChangesAsync();
+            db.ProductStoreSections.Add(new ProductStoreSection
+            {
+                FamilyId = familyId,
+                StoreId = store.Id,
+                ProductId = p.Id,
+                StoreSectionId = s.Id,
+            });
+            await db.SaveChangesAsync();
+            return (p, s);
+        });
+
+        var body = await Client.GetFromJsonAsync<JsonElement>(
+            $"/api/products/global/{product.Id}/section?storeId={store.Id}");
+
+        await Assert.That(body.GetProperty("sectionId").GetInt32()).IsEqualTo(section.Id);
+    }
+
+    [Test]
+    public async Task Get_global_product_section_returns_null_when_no_memory()
+    {
+        var store = await Data.CreateStoreAsync();
+        var product = await UseDbAsync(async db =>
+        {
+            var p = new Product { Name = "Unknown-Lookup" };
+            db.Products.Add(p);
+            await db.SaveChangesAsync();
+            return p;
+        });
+
+        var body = await Client.GetFromJsonAsync<JsonElement>(
+            $"/api/products/global/{product.Id}/section?storeId={store.Id}");
+
+        await Assert.That(body.GetProperty("sectionId").ValueKind).IsEqualTo(JsonValueKind.Null);
+    }
+
+    [Test]
+    public async Task Get_global_product_section_is_scoped_to_store()
+    {
+        var familyId = await GetFamilyIdAsync();
+        var storeA = await Data.CreateStoreAsync(b => b.Named("A"));
+        var storeB = await Data.CreateStoreAsync(b => b.Named("B"));
+        var product = await UseDbAsync(async db =>
+        {
+            var p = new Product { Name = "Apples-Lookup" };
+            var sectionA = new StoreSection { FamilyId = familyId, StoreId = storeA.Id, Name = "Produce A", SortOrder = 1 };
+            db.Products.Add(p);
+            db.StoreSections.Add(sectionA);
+            await db.SaveChangesAsync();
+            db.ProductStoreSections.Add(new ProductStoreSection
+            {
+                FamilyId = familyId,
+                StoreId = storeA.Id,
+                ProductId = p.Id,
+                StoreSectionId = sectionA.Id,
+            });
+            await db.SaveChangesAsync();
+            return p;
+        });
+
+        // Lookup at store B — no memory there even though store A has one.
+        var body = await Client.GetFromJsonAsync<JsonElement>(
+            $"/api/products/global/{product.Id}/section?storeId={storeB.Id}");
+
+        await Assert.That(body.GetProperty("sectionId").ValueKind).IsEqualTo(JsonValueKind.Null);
+    }
+
+    [Test]
+    public async Task Get_family_product_section_returns_remembered_section()
+    {
+        var familyId = await GetFamilyIdAsync();
+        var store = await Data.CreateStoreAsync();
+        var (product, section) = await UseDbAsync(async db =>
+        {
+            var p = new FamilyProduct { FamilyId = familyId, Name = "House Bread" };
+            var s = new StoreSection { FamilyId = familyId, StoreId = store.Id, Name = "Bakery", SortOrder = 1 };
+            db.FamilyProducts.Add(p);
+            db.StoreSections.Add(s);
+            await db.SaveChangesAsync();
+            db.ProductStoreSections.Add(new ProductStoreSection
+            {
+                FamilyId = familyId,
+                StoreId = store.Id,
+                FamilyProductId = p.Id,
+                StoreSectionId = s.Id,
+            });
+            await db.SaveChangesAsync();
+            return (p, s);
+        });
+
+        var body = await Client.GetFromJsonAsync<JsonElement>(
+            $"/api/products/family/{product.Id}/section?storeId={store.Id}");
+
+        await Assert.That(body.GetProperty("sectionId").GetInt32()).IsEqualTo(section.Id);
+    }
+
+    [Test]
+    public async Task Get_family_product_section_returns_404_for_other_family_product()
+    {
+        var store = await Data.CreateStoreAsync();
+        var otherFamilyProductId = await UseDbAsync(async db =>
+        {
+            var otherFamily = new Family { Name = "Other", InviteCode = "other-code" };
+            db.Families.Add(otherFamily);
+            await db.SaveChangesAsync();
+            var p = new FamilyProduct { FamilyId = otherFamily.Id, Name = "Their Bread" };
+            db.FamilyProducts.Add(p);
+            await db.SaveChangesAsync();
+            return p.Id;
+        });
+
+        var response = await Client.GetAsync(
+            $"/api/products/family/{otherFamilyProductId}/section?storeId={store.Id}");
+
+        await Assert.That(response.StatusCode).IsEqualTo(HttpStatusCode.NotFound);
+    }
 }

src/backend/YesChef.Api.IntegrationTests/Features/RecipeEndpointsTests.cs

@@ -111,7 +111,7 @@ public class RecipeEndpointsTests : AuthenticatedIntegrationTest
         await Assert.That(body.GetProperty("title").GetString()).IsEqualTo("Soup");
         await Assert.That(body.GetProperty("createdBy").GetString()).IsEqualTo(User.Name);
         var ingredientNames = body.GetProperty("ingredients").EnumerateArray()
-            .Select(i => i.GetProperty("name").GetString()).ToArray();
+            .Select(i => i.GetProperty("name").GetString()!).ToArray();
         await Assert.That(ingredientNames).IsEquivalentTo(new[] { "broth cube", "water" });
     }
 
@@ -131,7 +131,7 @@ public class RecipeEndpointsTests : AuthenticatedIntegrationTest
 
         var hits = await Client.GetFromJsonAsync<List<JsonElement>>("/api/recipes?q=Pa");
 
-        await Assert.That(hits!.Select(h => h.GetProperty("title").GetString()))
+        await Assert.That(hits!.Select(h => h.GetProperty("title").GetString()!))
             .IsEquivalentTo(new[] { "Pancakes", "Pad Thai" });
     }
 

src/backend/YesChef.Api.IntegrationTests/Features/ShoppingListEndpointsTests.cs

@@ -77,7 +77,7 @@ public class ShoppingListEndpointsTests : AuthenticatedIntegrationTest
 
         await Assert.That(body.GetProperty("name").GetString()).IsEqualTo("groceries");
         var items = body.GetProperty("items").EnumerateArray()
-            .Select(i => i.GetProperty("name").GetString()).ToArray();
+            .Select(i => i.GetProperty("name").GetString()!).ToArray();
         await Assert.That(items).IsEquivalentTo(new[] { "bread", "milk" });
     }
 
@@ -377,7 +377,7 @@ public class ShoppingListEndpointsTests : AuthenticatedIntegrationTest
 
         var body = await Client.GetFromJsonAsync<JsonElement>($"/api/lists/{list.Id}");
         var items = body.GetProperty("items").EnumerateArray()
-            .Select(i => i.GetProperty("name").GetString()).ToArray();
+            .Select(i => i.GetProperty("name").GetString()!).ToArray();
         await Assert.That(items).IsEquivalentTo(new[] { "kept" });
     }
 

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")

src/backend/YesChef.Api/Entities/UnitCategory.cs

@@ -1,5 +1,8 @@
+using System.Text.Json.Serialization;
+
 namespace YesChef.Api.Entities;
 
+[JsonConverter(typeof(JsonStringEnumConverter<UnitCategory>))]
 public enum UnitCategory
 {
     Count = 0,

src/backend/YesChef.Api/Entities/UnitCategoryFlags.cs

@@ -1,3 +1,5 @@
+using System.Text.Json.Serialization;
+
 namespace YesChef.Api.Entities;
 
 /// <summary>
@@ -7,6 +9,7 @@ namespace YesChef.Api.Entities;
 /// 32-bit integer so families can OR in additional categories over time.
 /// </summary>
 [System.Flags]
+[JsonConverter(typeof(JsonStringEnumConverter<UnitCategoryFlags>))]
 public enum UnitCategoryFlags
 {
     None = 0,

src/backend/YesChef.Api/Features/Products/ProductEndpoints.cs

@@ -1,7 +1,9 @@
+using System.Text.Json.Serialization;
 using Microsoft.EntityFrameworkCore;
 using YesChef.Api.Auth;
 using YesChef.Api.Data;
 using YesChef.Api.Entities;
+using YesChef.Api.Features.ShoppingLists;
 
 namespace YesChef.Api.Features.Products;
 
@@ -10,6 +12,7 @@ public static class ProductEndpoints
     /// <summary>Discriminator on product DTOs so the client can route subsequent
     /// PUT/POST calls to the right code path. The catalog UI itself doesn't
     /// need to surface this — both kinds render the same way.</summary>
+    [JsonConverter(typeof(JsonStringEnumConverter<ProductKind>))]
     public enum ProductKind { Global, Family }
 
     public record ProductDto(
@@ -239,6 +242,30 @@ public static class ProductEndpoints
                 ovr.AllowedUnitCategories ?? product.AllowedUnitCategories));
         });
 
+        // Look up the per-store section memory for a product so the add-item
+        // UI can pre-fill the dropdown the moment the user picks a product
+        // from the typeahead. The backend already applies this memory on POST
+        // when no section is supplied — this endpoint just lets the client
+        // mirror that decision so the user can see (and override) it.
+        // Returns { sectionId: int | null }. No memory → null, not 404.
+        group.MapGet("/global/{id:int}/section", async (int id, int storeId, YesChefDb db, HttpContext http) =>
+        {
+            var familyId = http.User.GetFamilyId();
+            var sectionId = await ShoppingListEndpoints.LookupRememberedSectionAsync(db, familyId, storeId, id, null);
+            return Results.Ok(new { sectionId });
+        });
+
+        group.MapGet("/family/{id:int}/section", async (int id, int storeId, YesChefDb db, HttpContext http) =>
+        {
+            var familyId = http.User.GetFamilyId();
+            // FamilyProducts are tenant-scoped — refuse to leak even the
+            // existence of another family's product id.
+            if (!await db.FamilyProducts.AnyAsync(p => p.Id == id && p.FamilyId == familyId))
+                return Results.NotFound();
+            var sectionId = await ShoppingListEndpoints.LookupRememberedSectionAsync(db, familyId, storeId, null, id);
+            return Results.Ok(new { sectionId });
+        });
+
         return group;
     }
 

src/backend/YesChef.Api/Features/Units/UnitEndpoints.cs

@@ -1,3 +1,4 @@
+using System.Text.Json.Serialization;
 using Microsoft.EntityFrameworkCore;
 using YesChef.Api.Auth;
 using YesChef.Api.Data;
@@ -9,6 +10,7 @@ public static class UnitEndpoints
 {
     /// <summary>Discriminator so the client can route subsequent PUT/DELETE
     /// calls to the right code path. Global units cannot be edited.</summary>
+    [JsonConverter(typeof(JsonStringEnumConverter<UnitKind>))]
     public enum UnitKind { Global, Family }
 
     public record UnitDto(

src/frontend/src/routes/lists/[id]/+page.svelte

@@ -56,6 +56,9 @@
 	let newItemAllowedUnitCategories = $state(0);
 	let loading = $state(true);
 	let connection: HubConnection | null = null;
+	// Seq guard so a slow section lookup can't overwrite the section a
+	// later (faster) product pick already set.
+	let sectionLookupSeq = 0;
 
 	const listId = $derived(Number(page.params.id));
 	const uncheckedItems = $derived(items.filter((i) => !i.isChecked));
@@ -203,19 +206,39 @@
 		}
 	}
 
-	function onItemProductChange(product: ProductSuggestion | null) {
+	async function onItemProductChange(product: ProductSuggestion | null) {
+		const seq = ++sectionLookupSeq;
 		if (product === null) {
 			newItemProductId = null;
 			newItemFamilyProductId = null;
 			newItemAllowedUnitCategories = 0;
-		} else if (product.kind === 'Global') {
+			// Don't clobber the user's section choice when they edit the
+			// name and the link clears — they may have set it deliberately.
+			return;
+		}
+		if (product.kind === 'Global') {
 			newItemProductId = product.id;
 			newItemFamilyProductId = null;
-			newItemAllowedUnitCategories = product.allowedUnitCategories;
 		} else {
 			newItemProductId = null;
 			newItemFamilyProductId = product.id;
-			newItemAllowedUnitCategories = product.allowedUnitCategories;
+		}
+		newItemAllowedUnitCategories = product.allowedUnitCategories;
+
+		// Mirror the backend's auto-assign rule: pre-fill the section dropdown
+		// from per-store memory so the user can see (and override) what's
+		// about to happen. Null result means no memory — reset to "No section".
+		if (!list) return;
+		const path = product.kind === 'Global' ? 'global' : 'family';
+		try {
+			const res = await api<{ sectionId: number | null }>(
+				`/api/products/${path}/${product.id}/section?storeId=${list.store.id}`
+			);
+			if (seq !== sectionLookupSeq) return;
+			newItemSectionId = res.sectionId;
+		} catch {
+			// Lookup failure shouldn't block adding an item — leave the
+			// section alone and let the backend auto-assign on POST.
 		}
 	}