fix(api): replace threading.Lock with asyncio.Lock for hydration guard

await inside a threading.Lock yields to the event loop while the OS
thread still holds the lock — potential deadlock under FastAPI thread
pool dispatch. asyncio.Lock is the correct primitive for async
critical sections. Also fixed stale diurnal.py docstring that had the
delegation direction backwards.

decnet/realism/diurnal.py

@@ -38,7 +38,7 @@ def _parse_window(window: str) -> tuple[int, int, int, int] | None:
 
     Returns ``None`` for malformed input — callers treat that as
     "always-on" so a single config typo never silences the whole fleet
-    (mirrors :func:`decnet.realism.personas.in_active_hours` semantics).
+    (:func:`decnet.realism.personas.in_active_hours` delegates here).
     """
     try:
         start_s, end_s = window.split("-")