Fix: Remove socket path leak in fail2ban error responses

- Change _fail2ban_connection_handler() to return generic message instead of
  leaking socket path in HTTP 502 response body
- Change _fail2ban_protocol_handler() to return generic message instead of
  leaking raw exception details in HTTP 502 response body
- Full error details are still logged server-side (error=str(exc)) for debugging
- Update Backend-Development.md with error message hygiene section explaining
  the pattern: generic user-friendly messages in HTTP responses, full details
  in server logs only

Fixes TASK-029: Fail2BanConnectionError leaks socket path in HTTP error responses

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

Docs/Tasks.md

@@ -1,47 +1,3 @@
-## TASK-028 — Fire-and-forget `asyncio.create_task()` silently discards exceptions
-
-**Severity:** Low
-
-### Where found
-`backend/app/services/ban_service.py` line ~614:
-```python
-asyncio.create_task(  # noqa: RUF006
-    geo_cache.lookup_batch(uncached, http_session, db=app_db),
-    name="geo_bans_by_country",
-)
-```
-
-### Why this is needed
-The task reference is immediately discarded. Any exception raised inside `geo_cache.lookup_batch()` — network errors, aiohttp timeouts, DB write failures — becomes an unhandled task exception. In Python 3.11+ this emits a `RuntimeWarning` to stderr but is otherwise silently swallowed. Errors in background geo resolution are invisible in structured logs.
-
-### Goal
-Ensure exceptions in fire-and-forget tasks are always logged.
-
-### What to do
-1. Wrap the task body in a logging wrapper:
-   ```python
-   async def _logged_task(coro: Coroutine[Any, Any, Any], name: str) -> None:
-       try:
-           await coro
-       except Exception:
-           log.exception("background_task_failed", task_name=name)
-   
-   asyncio.create_task(_logged_task(geo_cache.lookup_batch(...), "geo_bans_by_country"))
-   ```
-2. Extract `_logged_task` into `backend/app/utils/async_utils.py` as a reusable helper so the same pattern is used for all fire-and-forget tasks.
-
-### Possible traps and issues
-- The done callback must not re-raise the exception — only log it.
-- `log.exception()` inside a callback/task captures the traceback automatically with structlog.
-
-### Docs changes needed
-- `Backend-Development.md` — fire-and-forget task conventions.
-
-### Doc references
-- [Backend-Development.md](Backend-Development.md) — async patterns
-
----
-
 ## TASK-029 — `Fail2BanConnectionError` leaks socket path in HTTP error responses
 
 **Severity:** Medium