refactor: improve backend type safety and import organization

- Add TYPE_CHECKING guards for runtime-expensive imports (aiohttp, aiosqlite)
- Reorganize imports to follow PEP 8 conventions
- Convert TypeAlias to modern PEP 695 type syntax (where appropriate)
- Use Sequence/Mapping from collections.abc for type hints (covariant)
- Replace string literals with cast() for improved type inference
- Fix casting of Fail2BanResponse and TypedDict patterns
- Add IpLookupResult TypedDict for precise return type annotation
- Reformat overlong lines for readability (120 char limit)
- Add asyncio_mode and filterwarnings to pytest config
- Update test fixtures with improved type hints

This improves mypy type checking and makes type relationships explicit.

backend/tests/test_services/test_jail_service.py

@@ -996,9 +996,6 @@ class TestGetJailBannedIps:
 
     async def test_unknown_jail_raises_jail_not_found_error(self) -> None:
         """get_jail_banned_ips raises JailNotFoundError for unknown jail."""
-        responses = {
-            "status|ghost|short": (0, pytest.raises),  # will be overridden
-        }
         # Simulate fail2ban returning an "unknown jail" error.
         class _FakeClient:
             def __init__(self, **_kw: Any) -> None: