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/config.py

@@ -85,4 +85,4 @@ def get_settings() -> Settings:
         A validated :class:`Settings` object. Raises :class:`pydantic.ValidationError`
         if required keys are absent or values fail validation.
     """
-    return Settings()  # pydantic-settings populates required fields from env vars
+    return Settings()  # type: ignore[call-arg]  # pydantic-settings populates required fields from env vars