Offload ensure_jail_configs to a thread during startup

Use run_blocking for synchronous jail config file creation in backend/app/startup.py and mark Task 20 completed in Docs/Tasks.md.

Docs/Tasks.md

@@ -550,6 +550,8 @@ If the cookie name ever needs to change (e.g., to support multi-tenant deploymen
 
 ### Task 19 — Remove dead loop variable and asyncio import in auth_service
 
+**Status:** Completed
+
 **Severity:** Low
 
 **Where:**
@@ -578,6 +580,8 @@ Dead code misleads readers into thinking `loop` is used for something. The origi
 
 ### Task 20 — Offload ensure_jail_configs to a thread at startup
 
+**Status:** Completed
+
 **Severity:** Low
 
 **Where:**