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/app/services/blocklist_service.py

@@ -542,7 +542,7 @@ async def list_import_logs(
 # ---------------------------------------------------------------------------
 
 
-def _aiohttp_timeout(seconds: float) -> "aiohttp.ClientTimeout":
+def _aiohttp_timeout(seconds: float) -> aiohttp.ClientTimeout:
     """Return an :class:`aiohttp.ClientTimeout` with the given total timeout.
 
     Args: