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/config_service.py

@@ -95,7 +95,7 @@ def _ok(response: object) -> object:
         ValueError: If the return code indicates an error.
     """
     try:
-        code, data = cast(Fail2BanResponse, response)
+        code, data = cast("Fail2BanResponse", response)
     except (TypeError, ValueError) as exc:
         raise ValueError(f"Unexpected fail2ban response shape: {response!r}") from exc
     if code != 0:
@@ -128,7 +128,7 @@ def _ensure_list(value: object | None) -> list[str]:
     return [str(value)]
 
 
-_T = TypeVar("_T")
+T = TypeVar("T")
 
 
 async def _safe_get(
@@ -143,13 +143,13 @@ async def _safe_get(
         return default
 
 
-async def _safe_get_typed(
+async def _safe_get_typed[T](
     client: Fail2BanClient,
     command: Fail2BanCommand,
-    default: _T,
-) -> _T:
+    default: T,
+) -> T:
     """Send a command and return the result typed as ``default``'s type."""
-    return cast(_T, await _safe_get(client, command, default))
+    return cast("T", await _safe_get(client, command, default))
 
 
 def _is_not_found_error(exc: Exception) -> bool: