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/utils/async_utils.py

@@ -8,12 +8,17 @@ from __future__ import annotations
 
 import asyncio
 import functools
+from collections.abc import Callable, Coroutine
 from concurrent.futures import ThreadPoolExecutor
-from typing import Callable, ParamSpec, TypeVar
+from typing import Any, ParamSpec, TypeVar
+
+import structlog
 
 P = ParamSpec("P")
 T = TypeVar("T")
 
+log: structlog.stdlib.BoundLogger = structlog.get_logger()
+
 DEFAULT_BLOCKING_EXECUTOR: ThreadPoolExecutor = ThreadPoolExecutor(
     max_workers=16,
     thread_name_prefix="bangui-blocking",
@@ -43,3 +48,27 @@ async def run_blocking(
         func = functools.partial(func, *args, **kwargs)
         return await loop.run_in_executor(executor, func)
     return await loop.run_in_executor(executor, func, *args)
+
+
+async def logged_task(
+    coro: Coroutine[Any, Any, Any],
+    name: str,
+) -> None:
+    """Execute a coroutine with automatic exception logging.
+
+    Wraps fire-and-forget tasks to ensure exceptions are always logged and
+    do not become unhandled task exceptions. Use with asyncio.create_task():
+
+        asyncio.create_task(
+            logged_task(some_coroutine(), "task_name"),
+            name="task_name"
+        )
+
+    Args:
+        coro: Coroutine to execute.
+        name: Task name for logging context.
+    """
+    try:
+        await coro
+    except Exception:  # noqa: BLE001
+        log.exception("background_task_failed", task_name=name)