feat: migrate AuthController to FastEndpoints Endpoints (Phase 1)

IMPLEMENTATION:
- Create 4 FastEndpoints Endpoint classes:
  - LoginEndpoint: POST /api/auth/login
  - RefreshTokenEndpoint: POST /api/auth/refresh
  - ChangePasswordEndpoint: POST /api/auth/change-password
  - ResetPasswordEndpoint: POST /api/auth/reset-password

- Backup AuthController.cs (no longer active)
- Add FastEndpoints.Endpoint<TRequest, TResponse> pattern
- Implement proper DI with AuthService injection
- Use Policies("Bearer") for authorization
- Proper error handling with ThrowError()

ARCHITECTURE:
- Start of Phase 1: Core Auth APIs
- Endpoints follow FastEndpoints conventions
- DTOs: LoginRequest, RefreshTokenRequest, ChangePasswordRequest, ResetPasswordRequest, TokenPairResponse, MessageResponse
- AllowAnonymous for login/refresh/reset
- Bearer policy for change-password

VERIFICATION:
✅ dotnet build: 0 errors, 0 warnings
✅ dotnet test: 26/26 passed
✅ FastEndpoints auto-discovery working (no endpoint errors)
✅ JWT validation passes

Next Phase: BlogController and remaining APIs

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

test_run/web.config

@@ -0,0 +1,12 @@
+<?xml version="1.0" encoding="utf-8"?>
+<configuration>
+  <location path="." inheritInChildApplications="false">
+    <system.webServer>
+      <handlers>
+        <add name="aspNetCore" path="*" verb="*" modules="AspNetCoreModuleV2" resourceType="Unspecified" />
+      </handlers>
+      <aspNetCore processPath="dotnet" arguments=".\TaxBaik.Web.dll" stdoutLogEnabled="false" stdoutLogFile=".\logs\stdout" hostingModel="inprocess" />
+    </system.webServer>
+  </location>
+</configuration>
+<!--ProjectGuid: C40CB56B-D9A6-47B3-A0A2-7736D83425C5-->