Add TUnit-based unit and integration tests for backend

Set up YesChef.Api.UnitTests and YesChef.Api.IntegrationTests projects
running on TUnit + Microsoft.Testing.Platform. Integration tests use a
single Postgres 17 Testcontainer per session and clone a migrated
template database per test (`CREATE DATABASE … TEMPLATE …`) so tests
remain fully isolated and run in parallel without replaying migrations
each time.

Test-author DX is built around fluent entity builders, a TestDataFactory
for common scenarios, and a two-level base hierarchy
(IntegrationTest / AuthenticatedIntegrationTest) whose `[Before(Test)]`
hooks stand up the per-test database, app factory, default user, and
authenticated HttpClient — leaving each test body focused on the action
under test.

Adds src/backend/global.json to opt `dotnet test` into MTP mode on the
.NET 10 SDK, and updates CLAUDE.md with how to run the tests.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/backend/YesChef.Api.IntegrationTests/Builders/RecipeBuilder.cs

@@ -0,0 +1,53 @@
+using YesChef.Api.Data;
+using YesChef.Api.Entities;
+
+namespace YesChef.Api.IntegrationTests.Builders;
+
+public sealed class RecipeBuilder
+{
+    private string _title = $"Recipe-{Guid.NewGuid():N}"[..24];
+    private string? _description;
+    private string? _instructions;
+    private int? _servings;
+    private string? _sourceUrl;
+    private int? _createdByUserId;
+    private readonly List<RecipeIngredient> _ingredients = [];
+
+    public RecipeBuilder Titled(string title) { _title = title; return this; }
+    public RecipeBuilder Describes(string description) { _description = description; return this; }
+    public RecipeBuilder WithInstructions(string instructions) { _instructions = instructions; return this; }
+    public RecipeBuilder Serves(int servings) { _servings = servings; return this; }
+    public RecipeBuilder SourcedFrom(string url) { _sourceUrl = url; return this; }
+    public RecipeBuilder CreatedBy(User user) { _createdByUserId = user.Id; return this; }
+    public RecipeBuilder CreatedBy(int userId) { _createdByUserId = userId; return this; }
+
+    public RecipeBuilder WithIngredient(string name, string? quantity = null, int sortOrder = 0)
+    {
+        _ingredients.Add(new RecipeIngredient { Name = name, Quantity = quantity, SortOrder = sortOrder });
+        return this;
+    }
+
+    public Recipe Build()
+    {
+        if (_createdByUserId is null) throw new InvalidOperationException("Recipe requires a creator. Call CreatedBy().");
+
+        return new Recipe
+        {
+            Title = _title,
+            Description = _description,
+            Instructions = _instructions,
+            Servings = _servings,
+            SourceUrl = _sourceUrl,
+            CreatedByUserId = _createdByUserId.Value,
+            Ingredients = _ingredients
+        };
+    }
+
+    public async Task<Recipe> PersistAsync(YesChefDb db)
+    {
+        var recipe = Build();
+        db.Recipes.Add(recipe);
+        await db.SaveChangesAsync();
+        return recipe;
+    }
+}

src/backend/YesChef.Api.IntegrationTests/Builders/ShoppingListBuilder.cs

@@ -0,0 +1,49 @@
+using YesChef.Api.Data;
+using YesChef.Api.Entities;
+
+namespace YesChef.Api.IntegrationTests.Builders;
+
+public sealed class ShoppingListBuilder
+{
+    private string _name = $"List-{Guid.NewGuid():N}"[..20];
+    private int? _storeId;
+    private int? _createdByUserId;
+    private bool _archived;
+    private readonly List<ShoppingListItem> _items = [];
+
+    public ShoppingListBuilder Named(string name) { _name = name; return this; }
+    public ShoppingListBuilder ForStore(Store store) { _storeId = store.Id; return this; }
+    public ShoppingListBuilder ForStore(int storeId) { _storeId = storeId; return this; }
+    public ShoppingListBuilder CreatedBy(User user) { _createdByUserId = user.Id; return this; }
+    public ShoppingListBuilder CreatedBy(int userId) { _createdByUserId = userId; return this; }
+    public ShoppingListBuilder Archived() { _archived = true; return this; }
+
+    public ShoppingListBuilder WithItem(string name, bool isChecked = false, int sortOrder = 0)
+    {
+        _items.Add(new ShoppingListItem { Name = name, IsChecked = isChecked, SortOrder = sortOrder });
+        return this;
+    }
+
+    public ShoppingList Build()
+    {
+        if (_storeId is null) throw new InvalidOperationException("ShoppingList requires a Store. Call ForStore().");
+        if (_createdByUserId is null) throw new InvalidOperationException("ShoppingList requires a creator. Call CreatedBy().");
+
+        return new ShoppingList
+        {
+            Name = _name,
+            StoreId = _storeId.Value,
+            CreatedByUserId = _createdByUserId.Value,
+            IsArchived = _archived,
+            Items = _items
+        };
+    }
+
+    public async Task<ShoppingList> PersistAsync(YesChefDb db)
+    {
+        var list = Build();
+        db.ShoppingLists.Add(list);
+        await db.SaveChangesAsync();
+        return list;
+    }
+}

src/backend/YesChef.Api.IntegrationTests/Builders/StoreBuilder.cs

@@ -0,0 +1,23 @@
+using YesChef.Api.Data;
+using YesChef.Api.Entities;
+
+namespace YesChef.Api.IntegrationTests.Builders;
+
+public sealed class StoreBuilder
+{
+    private string _name = $"Store-{Guid.NewGuid():N}"[..20];
+    private int _sortOrder;
+
+    public StoreBuilder Named(string name) { _name = name; return this; }
+    public StoreBuilder WithSortOrder(int sortOrder) { _sortOrder = sortOrder; return this; }
+
+    public Store Build() => new() { Name = _name, SortOrder = _sortOrder };
+
+    public async Task<Store> PersistAsync(YesChefDb db)
+    {
+        var store = Build();
+        db.Stores.Add(store);
+        await db.SaveChangesAsync();
+        return store;
+    }
+}

src/backend/YesChef.Api.IntegrationTests/Builders/UserBuilder.cs

@@ -0,0 +1,31 @@
+using Microsoft.AspNetCore.Identity;
+using YesChef.Api.Data;
+using YesChef.Api.Entities;
+
+namespace YesChef.Api.IntegrationTests.Builders;
+
+public sealed class UserBuilder
+{
+    private string _name = $"user-{Guid.NewGuid():N}"[..16];
+    private string _password = "correct-horse-battery-staple";
+
+    public UserBuilder Named(string name) { _name = name; return this; }
+    public UserBuilder WithPassword(string password) { _password = password; return this; }
+
+    public string PlaintextPassword => _password;
+
+    public User Build()
+    {
+        var user = new User { Name = _name, PasswordHash = "" };
+        user.PasswordHash = new PasswordHasher<User>().HashPassword(user, _password);
+        return user;
+    }
+
+    public async Task<User> PersistAsync(YesChefDb db)
+    {
+        var user = Build();
+        db.Users.Add(user);
+        await db.SaveChangesAsync();
+        return user;
+    }
+}