Standardize async offloading behind shared executor helper

backend/app/services/log_service.py

@@ -7,6 +7,7 @@ fail2ban socket operations.
 from __future__ import annotations
 
 import asyncio
+from app.utils.async_utils import run_blocking
 import re
 from pathlib import Path
 
@@ -70,7 +71,7 @@ async def preview_log(req: LogPreviewRequest) -> LogPreviewResponse:
         )
 
     try:
-        raw_lines = await asyncio.get_event_loop().run_in_executor(
+        raw_lines = await run_blocking(
             None,
             _read_tail_lines,
             str(path),