Add a v3 task_transitions table (additive migration + startup assertion) and a
fail-soft, idempotent TransitionRecorder. build_graph gains an injected
transition_recorder that wraps every node via a functools.wraps'd _instrument
(signature-preserving so LangGraph still injects RunnableConfig); the coordinator
wires it. Records one row per node entry (idempotent under resume replay) and
closes the open row on terminal status. Backs the dashboard task-history view.
