fix: pass through Jellyfin error responses to client

- Modified PostJsonAsync to return error response body as JSON when available
- Updated AuthenticateByName to pass through Jellyfin's actual error response
- Clients now see Jellyfin's real error messages instead of generic ones

allstarr/Controllers/JellyfinController.cs

@@ -1833,9 +1833,17 @@ public class JellyfinController : ControllerBase
             // Forward to Jellyfin server with client headers
             var (result, statusCode) = await _proxyService.PostJsonAsync("Users/AuthenticateByName", body, Request.Headers);
             
-            if (result == null)
+            if (statusCode != 200)
             {
                 _logger.LogWarning("Authentication failed - status {StatusCode}", statusCode);
+                
+                // Pass through Jellyfin's error response if available
+                if (result != null)
+                {
+                    return StatusCode(statusCode, result.RootElement.GetRawText());
+                }
+                
+                // Fallback to generic error if no response body
                 if (statusCode == 401)
                 {
                     return Unauthorized(new { error = "Invalid username or password" });

allstarr/Services/Jellyfin/JellyfinProxyService.cs

@@ -374,6 +374,21 @@ public class JellyfinProxyService
             var errorContent = await response.Content.ReadAsStringAsync();
             _logger.LogWarning("❌ SESSION: Jellyfin POST request failed: {StatusCode} for {Url}. Response: {Response}", 
                 response.StatusCode, url, errorContent);
+            
+            // Try to parse error response as JSON to pass through to client
+            if (!string.IsNullOrWhiteSpace(errorContent))
+            {
+                try
+                {
+                    var errorDoc = JsonDocument.Parse(errorContent);
+                    return (errorDoc, statusCode);
+                }
+                catch
+                {
+                    // Not valid JSON, return null
+                }
+            }
+            
             return (null, statusCode);
         }