feat: add AI suggestion audit trail table

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

backend/alembic/env.py

@@ -16,6 +16,7 @@ from app.models.copilot_conversation import CopilotConversation
 from app.models.assistant_chat import AssistantChat
 from app.models.survey_response import SurveyResponse
 from app.models.survey_invite import SurveyInvite
+from app.models.ai_suggestion import AISuggestion  # noqa: F401
 from app.core.config import settings
 
 # this is the Alembic Config object