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/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;
+    }
+}