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>

Apis/api/Controllers/CaptchaController.cs

@@ -70,7 +70,7 @@ namespace Api.Controllers
                 {
                     Error = "Captcha verification failed.",
                     Code = "captcha_verification_failed",
-                    Score = verdict.Score
+                    Detail = verdict.Score.HasValue ? $"Score: {verdict.Score:0.00}" : null
                 });
             }
 

Apis/api/Controllers/CvMatcherController.cs

@@ -112,6 +112,16 @@ public sealed class CvMatcherController : ControllerBase
             _logger.LogWarning("CV upload proxy request was cancelled by the client.");
             return StatusCode(499, new ErrorResponse { Error = "Request cancelled.", Code = "request_cancelled" });
         }
+        catch (Refit.ApiException apiEx) when ((int)apiEx.StatusCode < 500)
+        {
+            // Forward upstream 4xx errors (e.g. "File is too large", "Only PDF files supported")
+            // so the browser can display the actionable message rather than a generic 502.
+            var body = await apiEx.GetContentAsAsync<ErrorResponse>();
+            _logger.LogWarning("Upstream cv-matcher-api returned {Status} during CV upload: {Error}",
+                (int)apiEx.StatusCode, body?.Error);
+            return StatusCode((int)apiEx.StatusCode,
+                body ?? new ErrorResponse { Error = apiEx.Message, Code = "upstream_error" });
+        }
         catch (Exception ex)
         {
             _logger.LogError(ex, "CV upload proxy request failed.");
@@ -196,6 +206,16 @@ public sealed class CvMatcherController : ControllerBase
             _logger.LogWarning("Job match proxy request was cancelled by the client.");
             return StatusCode(499, new ErrorResponse { Error = "Request cancelled.", Code = "request_cancelled" });
         }
+        catch (Refit.ApiException apiEx) when ((int)apiEx.StatusCode < 500)
+        {
+            // Forward upstream 4xx errors (e.g. "Could not extract enough job text",
+            // "Invalid job URL") so the browser can display the actionable message.
+            var body = await apiEx.GetContentAsAsync<ErrorResponse>();
+            _logger.LogWarning("Upstream cv-matcher-api returned {Status} during job match: {Error}",
+                (int)apiEx.StatusCode, body?.Error);
+            return StatusCode((int)apiEx.StatusCode,
+                body ?? new ErrorResponse { Error = apiEx.Message, Code = "upstream_error" });
+        }
         catch (Exception ex)
         {
             _logger.LogError(ex, "Job match proxy request failed.");

Apis/common/Responses/ErrorResponse.cs

@@ -1,9 +1,16 @@
 namespace Common.Responses;
 
+/// <summary>
+/// Standard error body returned by all API endpoints on 4xx and 5xx responses.
+/// </summary>
 public sealed class ErrorResponse
 {
+    /// <summary>Human-readable error message, safe to display directly to the end user for 4xx responses.</summary>
     public string Error { get; init; } = string.Empty;
+
+    /// <summary>Machine-readable error code for programmatic handling (e.g. <c>"captcha_verification_failed"</c>).</summary>
     public string? Code { get; init; }
+
+    /// <summary>Optional additional detail for debugging (not shown in UI).</summary>
     public string? Detail { get; init; }
-    public double? Score { get; init; }
 }