fix(memory): address critical bugs from multi-agent review

Bug Fixes:
- Remove singleton pattern from consolidation/reflection services to
  prevent stale database session bugs (session is now passed per-request)
- Add LRU eviction to MemoryToolService._working dict (max 1000 sessions)
  to prevent unbounded memory growth
- Replace O(n) list.remove() with O(1) OrderedDict.move_to_end() in
  RetrievalCache for better performance under load
- Use deque with maxlen for metrics histograms to prevent unbounded
  memory growth (circular buffer with 10k max samples)
- Use full UUID for checkpoint IDs instead of 8-char prefix to avoid
  collision risk at scale (birthday paradox at ~50k checkpoints)

Test Updates:
- Update checkpoint test to expect 36-char UUID
- Update reflection singleton tests to expect new factory behavior
- Add reset_memory_reflection() no-op for backwards compatibility

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

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

backend/app/services/memory/consolidation/service.py

@@ -892,27 +892,22 @@ class MemoryConsolidationService:
         return result
 
 
-# Singleton instance
-_consolidation_service: MemoryConsolidationService | None = None
-
-
+# Factory function - no singleton to avoid stale session issues
 async def get_consolidation_service(
     session: AsyncSession,
     config: ConsolidationConfig | None = None,
 ) -> MemoryConsolidationService:
     """
-    Get or create the memory consolidation service.
+    Create a memory consolidation service for the given session.
+
+    Note: This creates a new instance each time to avoid stale session issues.
+    The service is lightweight and safe to recreate per-request.
 
     Args:
-        session: Database session
+        session: Database session (must be active)
         config: Optional configuration
 
     Returns:
         MemoryConsolidationService instance
     """
-    global _consolidation_service
-    if _consolidation_service is None:
-        _consolidation_service = MemoryConsolidationService(
-            session=session, config=config
-        )
-    return _consolidation_service
+    return MemoryConsolidationService(session=session, config=config)