store only status-change events; UI client-side polling refactor

server/alembic/versions/0001_baseline.py

@@ -43,6 +43,7 @@ def upgrade() -> None:
             last_observed_at  TIMESTAMPTZ NOT NULL,
             last_event_id     UUID NOT NULL,
             incident_key      TEXT NOT NULL,
+            summary           TEXT,
             PRIMARY KEY (agent_id, check_id)
         );
         """