Fix error propagation: surface API validation messages in the UI

- UseJsonExceptionHandler now maps InvalidOperationException to 400 (was 500),
  so upstream business-rule rejections reach the browser as actionable messages.
- CvMatcherController forwards Refit 4xx bodies from cv-matcher-api instead
  of swallowing them in a generic 502.
- ErrorResponse.Score removed; CaptchaController puts the score in Detail.
- Frontend extractApiError helper reads the server Error/error/title field for
  4xx responses and falls back to a generic i18n string for 5xx / missing body.
- All four failure handlers (CV upload, CV match, contact form, subscribe form)
  updated to use extractApiError with the correct rate-limit i18n key.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Helpers/startup-helpers/StartupExtensions.cs

@@ -191,14 +191,35 @@ public static class StartupExtensions
             {
                 var feature = context.Features.Get<IExceptionHandlerFeature>();
                 var logger = context.RequestServices.GetRequiredService<ILoggerFactory>().CreateLogger(serviceName);
+
+                context.Response.ContentType = "application/json";
+
+                // InvalidOperationException signals an intentional business-rule violation
+                // (e.g. "Could not extract enough job text"). Surface it as 400 with the
+                // original message so the caller can show it directly to the user.
+                if (feature?.Error is InvalidOperationException ioe)
+                {
+                    logger.LogWarning(ioe, "Business rule violation in {Service}", serviceName);
+                    context.Response.StatusCode = StatusCodes.Status400BadRequest;
+                    await context.Response.WriteAsJsonAsync(new Common.Responses.ErrorResponse
+                    {
+                        Error = ioe.Message,
+                        Code = "validation_error"
+                    });
+                    return;
+                }
+
                 if (feature?.Error is not null)
                 {
                     logger.LogError(feature.Error, "Unhandled exception in {Service}", serviceName);
                 }
 
                 context.Response.StatusCode = StatusCodes.Status500InternalServerError;
-                context.Response.ContentType = "application/json";
-                await context.Response.WriteAsJsonAsync(new { error = "Unexpected server error." });
+                await context.Response.WriteAsJsonAsync(new Common.Responses.ErrorResponse
+                {
+                    Error = "Unexpected server error.",
+                    Code = "internal_error"
+                });
             });
         });
     }