Fix filename sanitization for Chinese titles in export

- Update _sanitize_filename() to preserve Unicode characters
- Use re.UNICODE flag to allow Chinese characters in filenames
- Clean up consecutive underscores and trim leading/trailing ones
- Fix 500 Internal Server Error when exporting with Chinese titles

src/server/services/export_service.py

@@ -20,8 +20,12 @@ def _sanitize_filename(title: str) -> str:
         return ""
     # Remove or replace invalid filename characters
     sanitized = re.sub(r'[<>:"/\\|?*]', '', title)
-    # Replace spaces and other characters with underscores
-    sanitized = re.sub(r'[^\w\-_\.]', '_', sanitized)
+    # Replace spaces and other problematic characters with underscores, but keep Unicode letters/numbers
+    sanitized = re.sub(r'[^\w\-_.]', '_', sanitized, flags=re.UNICODE)
+    # Remove multiple consecutive underscores
+    sanitized = re.sub(r'_+', '_', sanitized)
+    # Remove leading/trailing underscores
+    sanitized = sanitized.strip('_')
     # Limit length
     return sanitized[:50] if sanitized else ""