feat(api): ForbiddenProblemDetailsHandler 403 shape [UDT-006]

src/api/SIGCM2.Api/Authorization/ForbiddenProblemDetailsHandler.cs

@@ -0,0 +1,58 @@
+using System.Text.Json;
+using Microsoft.AspNetCore.Authorization;
+using Microsoft.AspNetCore.Authorization.Policy;
+
+namespace SIGCM2.Api.Authorization;
+
+/// <summary>
+/// Custom IAuthorizationMiddlewareResultHandler that emits a structured ProblemDetails
+/// response for 403 Forbidden outcomes (authenticated user, missing permission).
+///
+/// For 401 Unauthorized and successful outcomes, delegates to the default handler
+/// so the existing JWT Bearer challenge flow is unaffected (REQ-B-07).
+///
+/// Registered as singleton in Program.cs — depends only on framework services.
+/// </summary>
+public sealed class ForbiddenProblemDetailsHandler : IAuthorizationMiddlewareResultHandler
+{
+    private static readonly AuthorizationMiddlewareResultHandler DefaultHandler = new();
+
+    private static readonly JsonSerializerOptions SerializerOptions = new()
+    {
+        PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
+    };
+
+    public async Task HandleAsync(
+        RequestDelegate next,
+        HttpContext context,
+        AuthorizationPolicy policy,
+        PolicyAuthorizationResult authorizeResult)
+    {
+        // Only intercept 403s for authenticated users.
+        // If the user is not authenticated, the 401 challenge is handled by JwtBearer (REQ-B-07).
+        if (authorizeResult.Forbidden && context.User.Identity?.IsAuthenticated == true)
+        {
+            var requiredPermission = context.Items["RequiredPermission"] as string;
+
+            context.Response.StatusCode = StatusCodes.Status403Forbidden;
+            context.Response.ContentType = "application/problem+json; charset=utf-8";
+
+            var problem = new
+            {
+                type = "https://sigcm2.local/errors/forbidden",
+                title = "Acceso denegado",
+                status = 403,
+                detail = "No tenés el permiso requerido para ejecutar esta acción.",
+                permisoRequerido = requiredPermission,
+            };
+
+            await context.Response.WriteAsync(
+                JsonSerializer.Serialize(problem, SerializerOptions));
+
+            return;
+        }
+
+        // Delegate 401 challenges and successful outcomes to the default handler
+        await DefaultHandler.HandleAsync(next, context, policy, authorizeResult);
+    }
+}

src/api/SIGCM2.Api/Program.cs

@@ -26,6 +26,7 @@ builder.Services.AddInfrastructure(builder.Configuration);
 // Authorization — handler lives in Api layer; DO NOT move to Infrastructure DI
 builder.Services.AddAuthorization();
 builder.Services.AddScoped<IAuthorizationHandler, PermissionAuthorizationHandler>();
+builder.Services.AddSingleton<IAuthorizationMiddlewareResultHandler, ForbiddenProblemDetailsHandler>();
 
 // Controllers with exception filter
 builder.Services.AddControllers(opts =>