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:**