fix: add assembly configuration to FastEndpoints

PROBLEM:
- FastEndpoints was unable to find any endpoint declarations
- Caused 'System.InvalidOperationException' at startup
- Reason: AddFastEndpoints() was called without assembly configuration

SOLUTION:
- Add explicit assembly configuration to AddFastEndpoints()
- Specify config.Assemblies = new[] { typeof(Program).Assembly }
- Enables FastEndpoints to discover all endpoint classes in the assembly

VERIFICATION:
✅ dotnet build: 0 errors, 0 warnings
✅ dotnet test: 26/26 passed

This fixes the 'core dumped' issue where dotnet process was aborting
due to missing endpoint registration.

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

src/TaxBaik.Web/Program.cs

@@ -56,7 +56,10 @@ builder.Host.UseSerilog((context, config) =>
 
 // Controllers + FastEndpoints (API-First)
 builder.Services.AddControllers();
-builder.Services.AddFastEndpoints();
+builder.Services.AddFastEndpoints(config =>
+{
+    config.Assemblies = new[] { typeof(Program).Assembly };
+});
 builder.Services.AddProblemDetails();
 builder.Services.AddHealthChecks();
 builder.Services.AddRateLimiter(options =>