fix(memory): add thread-safe singleton initialization

- Add threading.Lock with double-check locking to ScopeManager
- Add asyncio.Lock with double-check locking to MemoryReflection
- Make reset_memory_metrics async with proper locking
- Update test fixtures to handle async reset functions

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/services/memory/scoping/scope.py

@@ -7,6 +7,7 @@ Global -> Project -> Agent Type -> Agent Instance -> Session
 """
 
 import logging
+import threading
 from dataclasses import dataclass, field
 from typing import Any, ClassVar
 from uuid import UUID
@@ -448,13 +449,24 @@ class ScopeManager:
         return False
 
 
-# Singleton manager instance
+# Singleton manager instance with thread-safe initialization
 _manager: ScopeManager | None = None
+_manager_lock = threading.Lock()
 
 
 def get_scope_manager() -> ScopeManager:
-    """Get the singleton scope manager instance."""
+    """Get the singleton scope manager instance (thread-safe)."""
     global _manager
     if _manager is None:
-        _manager = ScopeManager()
+        with _manager_lock:
+            # Double-check locking pattern
+            if _manager is None:
+                _manager = ScopeManager()
     return _manager
+
+
+def reset_scope_manager() -> None:
+    """Reset the scope manager singleton (for testing)."""
+    global _manager
+    with _manager_lock:
+        _manager = None