fix: add _processing_uids to prevent re-fetching emails being processed

src/email_client.py

@@ -163,9 +163,6 @@ def fetch_unseen_emails(account: EmailAccount) -> list[Email]:
                     emails.append(Email(uid=uid, subject=subject, sender=sender, recipient=recipient,
                                         body=body, date=date_str, reply_to=reply_to, account_email=account.username,
                                         folder=folder))
-
-                    # FETCH 可能被服务器自动标已读，立即标回未读
-                    conn.uid("STORE", uid, "-FLAGS", "\\Seen")
                 except Exception as e:
                     logger.error("    UID %s FETCH 失败，跳过: %s", uid, e)
                     continue