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/reflection/service.py

@@ -7,6 +7,7 @@ Implements pattern detection, success/failure analysis, anomaly detection,
 and insight generation.
 """
 
+import asyncio
 import logging
 import statistics
 from collections import Counter, defaultdict
@@ -1425,8 +1426,9 @@ class MemoryReflection:
         )
 
 
-# Singleton instance
+# Singleton instance with async-safe initialization
 _memory_reflection: MemoryReflection | None = None
+_reflection_lock = asyncio.Lock()
 
 
 async def get_memory_reflection(
@@ -1434,7 +1436,7 @@ async def get_memory_reflection(
     config: ReflectionConfig | None = None,
 ) -> MemoryReflection:
     """
-    Get or create the memory reflection service.
+    Get or create the memory reflection service (async-safe).
 
     Args:
         session: Database session
@@ -1445,11 +1447,15 @@ async def get_memory_reflection(
     """
     global _memory_reflection
     if _memory_reflection is None:
-        _memory_reflection = MemoryReflection(session=session, config=config)
+        async with _reflection_lock:
+            # Double-check locking pattern
+            if _memory_reflection is None:
+                _memory_reflection = MemoryReflection(session=session, config=config)
     return _memory_reflection
 
 
-def reset_memory_reflection() -> None:
-    """Reset the memory reflection singleton."""
+async def reset_memory_reflection() -> None:
+    """Reset the memory reflection singleton (async-safe)."""
     global _memory_reflection
-    _memory_reflection = None
+    async with _reflection_lock:
+        _memory_reflection = None