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/pyproject.toml

@@ -60,4 +60,5 @@ plugins = ["pydantic.mypy"]
 asyncio_mode = "auto"
 pythonpath = [".", "../fail2ban-master"]
 testpaths = ["tests"]
-addopts = "--cov=app --cov-report=term-missing"
+addopts = "--asyncio-mode=auto --cov=app --cov-report=term-missing"
+filterwarnings = ["ignore::pytest.PytestRemovedIn9Warning"]