TASK-028: Add exception logging to fire-and-forget asyncio.create_task()

- Create logged_task() helper in backend/app/utils/async_utils.py to wrap
  fire-and-forget coroutines with exception logging
- Ensures unhandled task exceptions are always logged to structlog instead of
  silently discarded (Python 3.11+ RuntimeWarning)
- Update ban_service.py to use logged_task() for geo_cache.lookup_batch()
  background resolution
- Add comprehensive tests for logged_task() in test_async_utils.py
- Document fire-and-forget task conventions in Backend-Development.md

The logged_task() wrapper catches any exception raised in a background task,
logs it with full traceback context and task name, and never re-raises.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/app/services/ban_service.py

@@ -41,6 +41,7 @@ from app.models.ban import (
 from app.repositories import fail2ban_db_repo
 from app.repositories import history_archive_repo as default_history_archive_repo
 from app.services.fail2ban_metadata_service import default_fail2ban_metadata_service
+from app.utils.async_utils import logged_task
 from app.utils.constants import (
     DEFAULT_PAGE_SIZE,
     FAIL2BAN_SOCKET_TIMEOUT,
@@ -611,8 +612,11 @@ async def bans_by_country(
             if geo_cache is not None:
                 # Fire-and-forget: lookup_batch handles rate-limiting / retries.
                 # The dirty-set flush task persists results to the DB.
-                asyncio.create_task(  # noqa: RUF006
-                    geo_cache.lookup_batch(uncached, http_session, db=app_db),
+                asyncio.create_task(
+                    logged_task(
+                        geo_cache.lookup_batch(uncached, http_session, db=app_db),
+                        "geo_bans_by_country",
+                    ),
                     name="geo_bans_by_country",
                 )
     elif geo_enricher is not None and unique_ips: