fix(memory): use deque for metrics histograms to ensure bounded memory usage

- Replace default empty list with `deque` for `memory_retrieval_latency_seconds`
- Prevents unbounded memory growth by leveraging bounded circular buffer behavior

backend/app/services/memory/metrics/collector.py

@@ -401,8 +401,8 @@ class MemoryMetrics:
             )
 
             # Calculate average latencies
-            retrieval_latencies = self._histograms.get(
+            retrieval_latencies = list(
-                "memory_retrieval_latency_seconds", []
+                self._histograms.get("memory_retrieval_latency_seconds", deque())
             )
             avg_retrieval_latency = (
                 sum(retrieval_latencies) / len(retrieval_latencies)