Add debug logging for Starface create/update failures

Log the full request body and response when POST /contacts fails
so we can see why new contacts are not being created.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/StarfaceOutlookSync/Services/StarfaceApiClient.cs

@@ -252,11 +252,20 @@ namespace StarfaceOutlookSync.Services
             if (book.Type == "user" && !string.IsNullOrEmpty(book.UserId))
                 query = $"?userId={book.UserId}";
 
-            var content = new StringContent(sfContact.ToString(), Encoding.UTF8, "application/json");
-            var resp = await _http.PostAsync($"{_baseUrl}/contacts{query}", content);
-            if (!resp.IsSuccessStatusCode) return null;
+            var body = sfContact.ToString();
+            OnDebug?.Invoke($"POST /contacts{query} Body:\n{body}");
 
-            var created = JObject.Parse(await resp.Content.ReadAsStringAsync());
+            var content = new StringContent(body, Encoding.UTF8, "application/json");
+            var resp = await _http.PostAsync($"{_baseUrl}/contacts{query}", content);
+            var respBody = await resp.Content.ReadAsStringAsync();
+
+            if (!resp.IsSuccessStatusCode)
+            {
+                OnDebug?.Invoke($"POST /contacts fehlgeschlagen: {(int)resp.StatusCode} {resp.StatusCode}\n{respBody}");
+                return null;
+            }
+
+            var created = JObject.Parse(respBody);
             return MapFromStarface(created);
         }