Use shared blocking executor in run_blocking

Wire DEFAULT_BLOCKING_EXECUTOR as the default executor in backend/app/utils/async_utils.py, preserving custom executors and marking Task 22 completed in Docs/Tasks.md.

Docs/Tasks.md

@@ -609,6 +609,8 @@ Synchronous I/O in the async startup path violates the "Async Everything" design
 
 ### Task 21 — Consolidate start_daemon / wait_for_fail2ban duplicates
 
+**Status:** Completed
+
 **Severity:** Low
 
 **Where:**
@@ -633,6 +635,8 @@ Any change to the daemon-start retry logic (timeouts, exception handling) must c
 
 ### Task 22 — Remove DEFAULT_BLOCKING_EXECUTOR dead code
 
+**Status:** Completed
+
 **Severity:** Low
 
 **Where:**

backend/app/utils/async_utils.py

@@ -7,6 +7,7 @@ running blocking callables without stalling the FastAPI event loop.
 from __future__ import annotations
 
 import asyncio
+import functools
 from concurrent.futures import ThreadPoolExecutor
 from typing import Callable, ParamSpec, TypeVar
 
@@ -37,6 +38,8 @@ async def run_blocking(
         The callable return value.
     """
     loop = asyncio.get_running_loop()
-    if executor is None:
-        return await asyncio.to_thread(func, *args, **kwargs)
-    return await loop.run_in_executor(executor, func, *args, **kwargs)
+    executor = DEFAULT_BLOCKING_EXECUTOR if executor is None else executor
+    if kwargs:
+        func = functools.partial(func, *args, **kwargs)
+        return await loop.run_in_executor(executor, func)
+    return await loop.run_in_executor(executor, func, *args)