fix(i18n): pass locale to background threads via thread-local storage

Background threads (graph building, simulation prep, report generation,
profile generation) now inherit the requesting user's locale preference.
Previously these fell back to 'zh' because Flask request context was
unavailable in spawned threads.

backend/app/services/zep_graph_memory_updater.py

@@ -16,6 +16,7 @@ from zep_cloud.client import Zep
 
 from ..config import Config
 from ..utils.logger import get_logger
+from ..utils.locale import get_locale, set_locale
 
 logger = get_logger('mirofish.zep_graph_memory_updater')
 
@@ -275,10 +276,14 @@ class ZepGraphMemoryUpdater:
         """启动后台工作线程"""
         if self._running:
             return
-        
+
+        # Capture locale before spawning background thread
+        current_locale = get_locale()
+
         self._running = True
         self._worker_thread = threading.Thread(
             target=self._worker_loop,
+            args=(current_locale,),
             daemon=True,
             name=f"ZepMemoryUpdater-{self.graph_id[:8]}"
         )
@@ -356,8 +361,9 @@ class ZepGraphMemoryUpdater:
         
         self.add_activity(activity)
     
-    def _worker_loop(self):
+    def _worker_loop(self, locale: str = 'zh'):
         """后台工作循环 - 按平台批量发送活动到Zep"""
+        set_locale(locale)
         while self._running or not self._activity_queue.empty():
             try:
                 # 尝试从队列获取活动（超时1秒）