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/graph_builder.py

@@ -17,7 +17,7 @@ from ..config import Config
 from ..models.task import TaskManager, TaskStatus
 from ..utils.zep_paging import fetch_all_nodes, fetch_all_edges
 from .text_processor import TextProcessor
-from ..utils.locale import t
+from ..utils.locale import t, get_locale, set_locale
 
 
 @dataclass
@@ -84,10 +84,13 @@ class GraphBuilderService:
             }
         )
         
+        # Capture locale before spawning background thread
+        current_locale = get_locale()
+
         # 在后台线程中执行构建
         thread = threading.Thread(
             target=self._build_graph_worker,
-            args=(task_id, text, ontology, graph_name, chunk_size, chunk_overlap, batch_size)
+            args=(task_id, text, ontology, graph_name, chunk_size, chunk_overlap, batch_size, current_locale)
         )
         thread.daemon = True
         thread.start()
@@ -102,9 +105,11 @@ class GraphBuilderService:
         graph_name: str,
         chunk_size: int,
         chunk_overlap: int,
-        batch_size: int
+        batch_size: int,
+        locale: str = 'zh'
     ):
         """图谱构建工作线程"""
+        set_locale(locale)
         try:
             self.task_manager.update_task(
                 task_id,