Why: issue #3948 identifies four correctness breakers when
GATEWAY_WORKERS > 1. Work item 1 adds a startup gate that refuses
to boot when database.backend is not postgres, giving operators a
clear error instead of silent SQLite write-lock corruption.
The gate runs inside langgraph_runtime() before
init_engine_from_config, so a misconfigured deploy never opens a
listener or writes to disk. Non-integer env values fall back to 1
so uvicorn's own validation is unaffected.
