fix(sync): log Mattermost user lookup response body on failure

Adds response body to the warning log when Mattermost user lookup fails,
making it easier to diagnose token/permission issues from logs.

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

BlazorApp/Services/ProvisioningService.cs

@@ -451,7 +451,9 @@ public class ProvisioningService
             var userResponse = await client.GetAsync($"/api/v4/users/username/{username}");
             if (!userResponse.IsSuccessStatusCode)
             {
-                _logger.LogWarning("Mattermost user lookup failed for {Username}: {Status}", username, userResponse.StatusCode);
+                var lookupBody = await userResponse.Content.ReadAsStringAsync();
+                _logger.LogWarning("Mattermost user lookup failed for {Username}: {Status} {Body}",
+                    username, userResponse.StatusCode, lookupBody);
                 return (false, $"User not found ({userResponse.StatusCode})");
             }