feat(memory): complete M.2-M.5 memory upgrade phases with tests

- M.2: ForgettingCurve, MemoryDecay, MemoryReinforcement (selective forgetting)
- M.3: DailyDigestGenerator, ReminderScheduler, ProactiveInformer (proactive reminders)
- M.4: MemoryExtractor with LLM-based memory extraction from conversations
- M.5: MemoryRecallInjector with token budget control for prompt injection
- All phases include comprehensive unit tests (109 tests passing)
- Updated checklist.md to mark all tasks complete

backend/app/models/memory.py

@@ -53,3 +53,8 @@ class UserMemory(BaseModel):
     importance_score = Column(Float, default=0.5)  # 重要性分数 0.0-1.0
     importance_level = Column(String(20), default="medium")  # high | medium | low
     associated_topics = Column(JSON, nullable=True)  # List of topic strings
+    # M.2: 遗忘曲线系统
+    decay_score = Column(Float, default=1.0)  # 0.0-1.0, higher=more remembered
+    is_archived = Column(Boolean, default=False)  # 是否已归档到冷存储
+    last_accessed_at = Column(DateTime, nullable=True)  # 上次访问时间（用于遗忘计算）
+    archive_at = Column(DateTime, nullable=True)  # 归档时间