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_ban_service_perf.py

@@ -114,13 +114,13 @@ async def _seed_f2b_db(path: str, n: int) -> list[str]:
 
 
 @pytest.fixture(scope="module")
-def event_loop_policy() -> None:  # type: ignore[misc]
+def event_loop_policy() -> None:
     """Use the default event loop policy for module-scoped fixtures."""
     return None
 
 
 @pytest.fixture(scope="module")
-async def perf_db_path(tmp_path_factory: Any) -> str:  # type: ignore[misc]
+async def perf_db_path(tmp_path_factory: Any) -> str:
     """Return the path to a fail2ban DB seeded with 10 000 synthetic bans.
 
     Module-scoped so the database is created only once for all perf tests.