refactor: migrate to FastEndpoints framework

ARCHITECTURE CHANGE:
- Replaced ASP.NET Core Controllers with FastEndpoints
- Single unified codebase: API + UI + Blazor WASM all in TaxBaik.Web
- FastEndpoints provides:
  * Convention-based routing (no attribute decorators)
  * Built-in validation (FluentValidation)
  * Better request/response mapping
  * Cleaner dependency injection

Program.cs:
- AddControllers() → AddFastEndpoints()
- MapControllers() → MapFastEndpoints()

Next: Migrate existing API controllers to FastEndpoints endpoints

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

src/TaxBaik.Web/TaxBaik.Web.csproj

@@ -12,6 +12,7 @@
   </PropertyGroup>
 
   <ItemGroup>
+    <PackageReference Include="FastEndpoints" Version="5.30.0" />
     <PackageReference Include="Microsoft.AspNetCore.Components.WebAssembly.Server" Version="10.0.9" />
     <PackageReference Include="MudBlazor" Version="6.10.0" />
     <PackageReference Include="BCrypt.Net-Next" Version="4.0.3" />