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_jails.py

@@ -12,6 +12,7 @@ from httpx import ASGITransport, AsyncClient
 from app.config import Settings
 from app.db import init_db
 from app.main import create_app
+from app.models.ban import JailBannedIpsResponse
 from app.models.jail import Jail, JailDetailResponse, JailListResponse, JailStatus, JailSummary
 
 # ---------------------------------------------------------------------------
@@ -801,17 +802,17 @@ class TestGetJailBannedIps:
     def _mock_response(
         self,
         *,
-        items: list[dict] | None = None,
+        items: list[dict[str, str | None]] | None = None,
         total: int = 2,
         page: int = 1,
         page_size: int = 25,
-    ) -> "JailBannedIpsResponse":  # type: ignore[name-defined]
+    ) -> JailBannedIpsResponse:
         from app.models.ban import ActiveBan, JailBannedIpsResponse
 
         ban_items = (
             [
                 ActiveBan(
-                    ip=item.get("ip", "1.2.3.4"),
+                    ip=item.get("ip") or "1.2.3.4",
                     jail="sshd",
                     banned_at=item.get("banned_at", "2025-01-01T10:00:00+00:00"),
                     expires_at=item.get("expires_at", "2025-01-01T10:10:00+00:00"),