Remove FluentValidation dependency, use plain .NET validation

Replace AbstractValidator-based request validators with a small
IModelValidator<T>/ValidationResult pattern (ported from BuyEngine's
Common namespace), wired into the MVC pipeline via a
ModelValidationActionFilter so controllers and response shape are
unaffected. Also copy over the Guard precondition helper.

src/api/MicCheck.Api/Program.cs

@@ -1,11 +1,10 @@
 using System.Text;
 using System.Threading.RateLimiting;
-using FluentValidation;
-using FluentValidation.AspNetCore;
 using MicCheck.Api.Common.Security.ApiKeys;
 using MicCheck.Api.Audit;
 using MicCheck.Api.Common.Security.Authentication;
 using MicCheck.Api.Common.Security.Authorization;
+using MicCheck.Api.Common.Validation;
 using MicCheck.Api.Data;
 using MicCheck.Api.Environments;
 using MicCheck.Api.Features;
@@ -42,7 +41,7 @@ try
               .WriteTo.Console());
 
     builder.Services.AddOpenApi();
-    builder.Services.AddControllers()
+    builder.Services.AddControllers(options => options.Filters.Add<ModelValidationActionFilter>())
         .AddJsonOptions(options =>
             options.JsonSerializerOptions.Converters.Add(
                 new System.Text.Json.Serialization.JsonStringEnumConverter()));
@@ -92,20 +91,11 @@ try
             limiter.QueueLimit = 0;
         }));
 
-    builder.Services.AddFluentValidationAutoValidation();
-    builder.Services.AddValidatorsFromAssemblyContaining<Program>();
+    builder.Services.AddModelValidatorsFromAssemblyContaining<Program>();
 
     builder.Services.Configure<ApiBehaviorOptions>(options =>
     {
-        options.InvalidModelStateResponseFactory = context =>
-        {
-            var errors = context.ModelState
-                .Where(e => e.Value?.Errors.Count > 0)
-                .ToDictionary(
-                    kvp => kvp.Key,
-                    kvp => kvp.Value!.Errors.Select(e => e.ErrorMessage).ToArray());
-            return new UnprocessableEntityObjectResult(new { errors });
-        };
+        options.InvalidModelStateResponseFactory = context => ValidationProblemResponseFactory.Create(context.ModelState);
     });
 
     builder.Services.AddMemoryCache();