Mark async socket handling task done and implement startup cleanup

backend/app/startup.py

@@ -7,6 +7,7 @@ in ``app.main`` delegates resource creation and task registration here.
 
 from __future__ import annotations
 
+from contextlib import suppress
 from pathlib import Path
 
 import aiohttp
@@ -101,13 +102,22 @@ async def startup_shared_resources(
     http_session: aiohttp.ClientSession = _create_http_session(settings)
     geo_service.init_geoip(settings.geoip_db_path)
 
-    scheduler: AsyncIOScheduler = AsyncIOScheduler(timezone="UTC")
-    scheduler.start()
+    scheduler: AsyncIOScheduler | None = None
+    try:
+        scheduler = AsyncIOScheduler(timezone="UTC")
+        scheduler.start()
 
-    health_check.register(app)
-    blocklist_import.register(app)
-    geo_cache_flush.register(app)
-    geo_re_resolve.register(app)
-    history_sync.register(app)
+        health_check.register(app)
+        blocklist_import.register(app)
+        geo_cache_flush.register(app)
+        geo_re_resolve.register(app)
+        history_sync.register(app)
 
-    return http_session, scheduler
+        return http_session, scheduler
+    except Exception:
+        with suppress(Exception):
+            await http_session.close()
+        if scheduler is not None:
+            with suppress(Exception):
+                scheduler.shutdown(wait=False)
+        raise

backend/app/utils/fail2ban_client.py

@@ -121,9 +121,8 @@ def _send_command_sync(
 ) -> object:
     """Send a command to fail2ban and return the parsed response.
 
-    This is a **synchronous** function intended to be called from within
-    :func:`asyncio.get_event_loop().run_in_executor` so that the event loop
-    is not blocked.
+    This is a **synchronous** function intended to be executed via
+    :func:`asyncio.to_thread` so that the event loop is not blocked.
 
     Transient ``OSError`` conditions (``EAGAIN``, ``ECONNREFUSED``,
     ``ENOBUFS``) are retried up to :data:`_RETRY_MAX_ATTEMPTS` times with
@@ -299,15 +298,13 @@ class Fail2BanClient:
 
         async with self._command_semaphore:
             log.debug("fail2ban_sending_command", command=command)
-            loop: asyncio.AbstractEventLoop = asyncio.get_event_loop()
             try:
-                response: object = await loop.run_in_executor(
-                None,
-                _send_command_sync,
-                self.socket_path,
-                command,
-                self.timeout,
-            )
+                response: object = await asyncio.to_thread(
+                    _send_command_sync,
+                    self.socket_path,
+                    command,
+                    self.timeout,
+                )
             except Fail2BanConnectionError:
                 log.warning(
                     "fail2ban_connection_error",