fix(backend): use timezone-aware UTC in memory modules (fix pytest DeprecationWarnings) (#1992)

* fix(backend): use timezone-aware UTC in memory modules

Replace datetime.utcnow() with datetime.now(timezone.utc) and a shared
utc_now_iso_z() helper so persisted ISO timestamps keep the trailing Z
suffix without triggering Python 3.12+ deprecation warnings.

Made-with: Cursor

* refactor(backend): use removesuffix for utc_now_iso_z suffix

Makes the +00:00 -> Z transform explicit for the trailing offset only
(Copilot review on PR #1992).

Made-with: Cursor

* style(backend): satisfy ruff UP017 with datetime.UTC in memory queue

Made-with: Cursor

---------

Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

backend/packages/harness/deerflow/agents/memory/storage.py

@@ -4,7 +4,7 @@ import abc
 import json
 import logging
 import threading
-from datetime import datetime
+from datetime import UTC, datetime
 from pathlib import Path
 from typing import Any
 
@@ -15,11 +15,16 @@ from deerflow.config.paths import get_paths
 logger = logging.getLogger(__name__)
 
 
+def utc_now_iso_z() -> str:
+    """Current UTC time as ISO-8601 with ``Z`` suffix (matches prior naive-UTC output)."""
+    return datetime.now(UTC).isoformat().removesuffix("+00:00") + "Z"
+
+
 def create_empty_memory() -> dict[str, Any]:
     """Create an empty memory structure."""
     return {
         "version": "1.0",
-        "lastUpdated": datetime.utcnow().isoformat() + "Z",
+        "lastUpdated": utc_now_iso_z(),
         "user": {
             "workContext": {"summary": "", "updatedAt": ""},
             "personalContext": {"summary": "", "updatedAt": ""},
@@ -137,7 +142,7 @@ class FileMemoryStorage(MemoryStorage):
 
         try:
             file_path.parent.mkdir(parents=True, exist_ok=True)
-            memory_data["lastUpdated"] = datetime.utcnow().isoformat() + "Z"
+            memory_data["lastUpdated"] = utc_now_iso_z()
 
             temp_path = file_path.with_suffix(".tmp")
             with open(temp_path, "w", encoding="utf-8") as f: