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("-")

decnet/web/router/realism/api_config.py

@@ -16,8 +16,8 @@ waiting for the orchestrator's next refresh tick.
 """
 from __future__ import annotations
 
+import asyncio
 import json
-import threading
 from typing import Any
 
 from fastapi import APIRouter, Depends, HTTPException
@@ -32,7 +32,7 @@ log = get_logger("api.realism.config")
 
 _CONFIG_KEY = "weights"
 _hydrated = False
-_hydrate_lock = threading.Lock()
+_hydrate_lock = asyncio.Lock()
 
 
 @router.get(
@@ -56,7 +56,7 @@ async def get_config(
     """
     global _hydrated
     if not _hydrated:
-        with _hydrate_lock:
+        async with _hydrate_lock:
             if not _hydrated:
                 row = await repo.get_realism_config(_CONFIG_KEY)
                 if row is not None: