feat: history cleanup (#7007)

fix AFF-1069

packages/backend/server/migrations/20240521100307_add_copilot_cost/migration.sql

@@ -0,0 +1,4 @@
+-- AlterTable
+ALTER TABLE "ai_sessions_metadata" ADD COLUMN     "deleted_at" TIMESTAMPTZ(6),
+ADD COLUMN     "messageCost" INTEGER NOT NULL DEFAULT 0,
+ADD COLUMN     "tokenCost" INTEGER NOT NULL DEFAULT 0;