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/Program.cs

@@ -50,8 +50,8 @@ builder.Host.UseSerilog((context, config) =>
     }
 });
 
-// Controllers (API)
-builder.Services.AddControllers();
+// FastEndpoints (API-First)
+builder.Services.AddFastEndpoints();
 builder.Services.AddProblemDetails();
 builder.Services.AddHealthChecks();
 builder.Services.AddRateLimiter(options =>
@@ -394,7 +394,7 @@ if (!app.Environment.IsDevelopment())
 }
 
 // API + Razor Pages + Blazor 매핑
-app.MapControllers();
+app.MapFastEndpoints();
 app.MapHealthChecks("/healthz");
 app.MapRazorPages();
 app.MapStaticAssets();