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/services/memory/__init__.py

@@ -3,11 +3,18 @@
 from app.services.memory.frequency_tracker import FrequencyTracker
 from app.services.memory.emotion_analyzer import EmotionAnalyzer
 from app.services.memory.impact_evaluator import ImpactEvaluator
-from app.services.memory.importance_scorer import ImportanceScorer
+from app.services.memory.importance_scorer import ImportanceScorer, ImportanceLevel
+from app.services.memory.forgetting_curve import ForgettingCurve
+from app.services.memory.memory_decay import MemoryDecay
+from app.services.memory.reinforcement import MemoryReinforcement
 
 __all__ = [
     "FrequencyTracker",
     "EmotionAnalyzer",
     "ImpactEvaluator",
     "ImportanceScorer",
+    "ImportanceLevel",
+    "ForgettingCurve",
+    "MemoryDecay",
+    "MemoryReinforcement",
 ]