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/metrics/collector.py

@@ -7,7 +7,7 @@ Collects and exposes metrics for the memory system.
 
 import asyncio
 import logging
-from collections import Counter, defaultdict
+from collections import Counter, defaultdict, deque
 from dataclasses import dataclass, field
 from datetime import UTC, datetime
 from enum import Enum
@@ -57,11 +57,17 @@ class MemoryMetrics:
     - Embedding operations
     """
 
+    # Maximum samples to keep in histogram (circular buffer)
+    MAX_HISTOGRAM_SAMPLES = 10000
+
     def __init__(self) -> None:
         """Initialize MemoryMetrics."""
         self._counters: dict[str, Counter[str]] = defaultdict(Counter)
         self._gauges: dict[str, dict[str, float]] = defaultdict(dict)
-        self._histograms: dict[str, list[float]] = defaultdict(list)
+        # Use deque with maxlen for bounded memory (circular buffer)
+        self._histograms: dict[str, deque[float]] = defaultdict(
+            lambda: deque(maxlen=self.MAX_HISTOGRAM_SAMPLES)
+        )
         self._histogram_buckets: dict[str, list[HistogramBucket]] = {}
         self._lock = asyncio.Lock()