feat: migrate InquiryController to FastEndpoints Endpoints (Phase 3)

IMPLEMENTATION:
- Create 7 FastEndpoints Endpoint classes for Inquiry API:
  - SubmitEndpoint: POST /api/inquiry (public)
  - GetPagedEndpoint: GET /api/inquiry (auth, paginated)
  - GetByIdEndpoint: GET /api/inquiry/{id} (auth)
  - UpdateStatusEndpoint: PUT /api/inquiry/{id}/status (auth)
  - UpdateAdminMemoEndpoint: PUT /api/inquiry/{id}/memo (auth)
  - UpdateEndpoint: PUT /api/inquiry/{id} (auth)
  - ConvertToClientEndpoint: POST /api/inquiry/{id}/convert-to-client (auth)

- Create InquiryDtos.cs with shared response types:
  - InquiryQuery (query parameters)
  - InquiryPagedResponse (paginated response)
  - UpdateStatusRequest, UpdateAdminMemoRequest, ConvertToClientRequest
  - ConvertToClientResponse, MessageResponse

- Backup InquiryController.cs (no longer active)

VERIFICATION:
✅ dotnet build: 0 errors, 0 warnings
✅ dotnet test: 26/26 passed
✅ Local service publish successful
✅ FastEndpoints auto-discovery working
✅ All 21 endpoints verified (Auth 4 + Blog 10 + Inquiry 7)

MIGRATION PROGRESS:
✅ Phase 1: Auth (4 endpoints) - DEPLOYED
✅ Phase 2: Blog (10 endpoints) - DEPLOYED
✅ Phase 3: Inquiry (7 endpoints) - READY FOR DEPLOYMENT

Next: Deploy Phase 3, then continue with remaining 13 Controllers

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

src/TaxBaik.Web/Endpoints/Inquiry/GetByIdEndpoint.cs

@@ -0,0 +1,32 @@
+using FastEndpoints;
+using TaxBaik.Application.Services;
+
+namespace TaxBaik.Web.Endpoints.Inquiry;
+
+public class GetByIdEndpoint : Endpoint<EmptyRequest, object>
+{
+    private readonly InquiryService _inquiryService;
+
+    public GetByIdEndpoint(InquiryService inquiryService)
+    {
+        _inquiryService = inquiryService;
+    }
+
+    public override void Configure()
+    {
+        Get("/api/inquiry/{id}");
+        Policies("Bearer");
+    }
+
+    public override async Task HandleAsync(EmptyRequest _, CancellationToken ct)
+    {
+        var id = Route<int>("id");
+        var inquiry = await _inquiryService.GetByIdAsync(id);
+        if (inquiry == null)
+        {
+            ThrowError("문의를 찾을 수 없습니다.", statusCode: 404);
+        }
+
+        await SendAsync(inquiry, 200, cancellation: ct);
+    }
+}