debug: add detailed network error logging to upload flush

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

AIStudyApp/AIStudyApp/Features/MaterialReader/ReadingEventUploadQueue.swift

@@ -262,6 +262,8 @@ final class ReadingEventUploadPipeline {
                 print("[UploadPipeline] Flush failed (\(error.statusCode ?? 0)): \(errorCode)")
             }
         } catch {
+            print("[UploadPipeline] Flush error: \(type(of: error)) - \(error.localizedDescription)")
+            print("[UploadPipeline] Full error: \(error)")
             queue.markRetry(ids: batch.map(\.id), errorCode: "NETWORK_ERROR")
         }
     }