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.slnx

@@ -1,3 +1,5 @@
 <Solution>
   <Project Path="YesChef.Api/YesChef.Api.csproj" />
+  <Project Path="YesChef.Api.UnitTests/YesChef.Api.UnitTests.csproj" />
+  <Project Path="YesChef.Api.IntegrationTests/YesChef.Api.IntegrationTests.csproj" />
 </Solution>