flowjet 2.1.8 — PostgreSQL checkpointer on the server surface

Fixed

• PostgreSQL checkpointer is now wired for the server surface.
  Checkpointers.open() (the /v1/responses + ACP server path) used to warn
  “Checkpointer backend 'postgresql' is not wired for the flowjet server surface
  yet” and yield None, so thread state never survived a turn — a deployment
  whose persistence.default_backend is postgresql silently ran every turn as
  a fresh conversation. It now builds an AsyncPostgresSaver on nano's shared
  checkpoints pool (resolve_checkpointer() → (None, pool)), opens the pool,
  and runs setup() once per process under nano's advisory lock
  (SharedCheckpointerPool.setup_checkpointer), so concurrent workers cannot
  race LangGraph's DDL. The shared pool is deliberately not closed on exit
  (it is process-wide). Failure still degrades to “no persistence” with a
  warning instead of failing startup; unknown backends keep the old behavior.