Poll Cli Event

Long-poll for the oldest unacked event; 204 when none arrives in time.

async def with every DB call in a threadpool, NOT a plain def: a sync route would occupy one of the threadpool's workers for the whole wait, and -- worse -- hold its pooled DB connection the entire time. The engine is sized pool_size=5, max_overflow=10 (core/database.py), so fifteen simultaneous listeners would starve every other request in the process. Between polls the session is rolled back, which returns its connection to the pool and starts a fresh READ COMMITTED snapshot, so newly committed events actually become visible.