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_setup.py

@@ -247,9 +247,9 @@ class TestSetupCompleteCaching:
         assert not getattr(app.state, "_setup_complete_cached", False)
 
         # First non-exempt request — middleware queries DB and sets the flag.
-        await client.post("/api/auth/login", json={"password": _SETUP_PAYLOAD["master_password"]})  # type: ignore[call-overload]
+        await client.post("/api/auth/login", json={"password": _SETUP_PAYLOAD["master_password"]})
 
-        assert app.state._setup_complete_cached is True  # type: ignore[attr-defined]
+        assert app.state._setup_complete_cached is True
 
     async def test_cached_path_skips_is_setup_complete(
         self,
@@ -267,12 +267,12 @@ class TestSetupCompleteCaching:
 
         # Do setup and warm the cache.
         await client.post("/api/setup", json=_SETUP_PAYLOAD)
-        await client.post("/api/auth/login", json={"password": _SETUP_PAYLOAD["master_password"]})  # type: ignore[call-overload]
-        assert app.state._setup_complete_cached is True  # type: ignore[attr-defined]
+        await client.post("/api/auth/login", json={"password": _SETUP_PAYLOAD["master_password"]})
+        assert app.state._setup_complete_cached is True
 
         call_count = 0
 
-        async def _counting(db):  # type: ignore[no-untyped-def]
+        async def _counting(db: aiosqlite.Connection) -> bool:
             nonlocal call_count
             call_count += 1
             return True