docs: add FastEndpoints framework guidelines to ENGINEERING_HARNESS

Documented FastEndpoints adoption:
- Framework: FastEndpoints v5.30.0
- Naming convention: Create[Entity]Endpoint, Get[Entity]Endpoint, etc.
- Location: Features/[DomainName]/ folder structure
- Validation: FluentValidation integration
- Coexistence: Controllers and FastEndpoints can run together
- URL routing: Explicit routes to maintain API contracts

Guidelines added to prevent URL conflicts and ensure consistent
endpoint implementation pattern across API layers.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

docs/ENGINEERING_HARNESS.md

@@ -88,6 +88,38 @@
 | Form | `XForm.razor` | 입력 컴포넌트와 UI validation |
 | Tests | unit + Playwright/API smoke | 회귀 방지 |
 
+## FastEndpoints Framework
+
+새 API 엔드포인트는 Controllers 대신 **FastEndpoints**로 구현한다.
+
+| 규칙 | 실행 |
+| --- | --- |
+| Library | `FastEndpoints` v5.30.0 이상 |
+| Naming | `Create[Entity]Endpoint`, `Get[Entity]Endpoint`, `List[Entity]Endpoint` 등 |
+| Location | `src/TaxBaik.Web/Features/[DomainName]/` |
+| Pattern | Request DTO → Endpoint class → Response DTO |
+| Validation | FluentValidation (FastEndpoints 내장) |
+| Registration | `builder.Services.AddFastEndpoints()` + `app.MapFastEndpoints()` |
+| Coexistence | Controllers와 FastEndpoints는 PathBase 내에서 병행 가능 (URL 충돌만 피함) |
+
+**주의**: 기존 Controllers에서 FastEndpoints로 마이그레이션 시, 기존 엔드포인트 URL(`/api/*`)이 변경되지 않도록 명시적 라우팅을 지정한다.
+
+```csharp
+public class GetBlogEndpoint : Endpoint<GetBlogRequest, GetBlogResponse>
+{
+    public override void Configure()
+    {
+        Get("/api/blog/{id}");
+        AllowAnonymous();
+    }
+
+    public override async Task HandleAsync(GetBlogRequest req, CancellationToken ct)
+    {
+        // Logic here
+    }
+}
+```
+
 ## Rendering Boundary
 
 | 영역 | 렌더링 | 데이터 접근 |