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_routers/test_auth.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+from collections.abc import Generator
 from unittest.mock import patch
 
 import pytest
@@ -157,12 +158,12 @@ class TestRequireAuthSessionCache:
     """In-memory session token cache inside ``require_auth``."""
 
     @pytest.fixture(autouse=True)
-    def reset_cache(self) -> None:  # type: ignore[misc]
+    def reset_cache(self) -> Generator[None, None, None]:
         """Flush the session cache before and after every test in this class."""
         from app import dependencies
 
         dependencies.clear_session_cache()
-        yield  # type: ignore[misc]
+        yield
         dependencies.clear_session_cache()
 
     async def test_second_request_skips_db(self, client: AsyncClient) -> None: