feat: add ai_chat_sessions database model and migration

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/models/__init__.py

@@ -28,6 +28,7 @@ from .maintenance_schedule import MaintenanceSchedule
 from .feedback import Feedback
 from .ai_conversation import AIConversation
 from .ai_usage import AIUsage
+from .ai_chat_session import AIChatSession
 
 __all__ = [
     "User",
@@ -67,4 +68,5 @@ __all__ = [
     "Feedback",
     "AIConversation",
     "AIUsage",
+    "AIChatSession",
 ]