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_tasks/test_health_check.py

@@ -270,7 +270,7 @@ class TestCrashDetection:
     async def test_crash_within_window_creates_pending_recovery(self) -> None:
         """An online→offline transition within 60 s of activation must set pending_recovery."""
         app = _make_app(prev_online=True)
-        now = datetime.datetime.now(tz=datetime.timezone.utc)
+        now = datetime.datetime.now(tz=datetime.UTC)
         app.state.last_activation = {
             "jail_name": "sshd",
             "at": now - datetime.timedelta(seconds=10),
@@ -297,7 +297,7 @@ class TestCrashDetection:
         app = _make_app(prev_online=True)
         app.state.last_activation = {
             "jail_name": "sshd",
-            "at": datetime.datetime.now(tz=datetime.timezone.utc)
+            "at": datetime.datetime.now(tz=datetime.UTC)
             - datetime.timedelta(seconds=120),
         }
         app.state.pending_recovery = None
@@ -315,8 +315,8 @@ class TestCrashDetection:
     async def test_came_online_marks_pending_recovery_resolved(self) -> None:
         """An offline→online transition must mark an existing pending_recovery as recovered."""
         app = _make_app(prev_online=False)
-        activated_at = datetime.datetime.now(tz=datetime.timezone.utc) - datetime.timedelta(seconds=30)
-        detected_at = datetime.datetime.now(tz=datetime.timezone.utc)
+        activated_at = datetime.datetime.now(tz=datetime.UTC) - datetime.timedelta(seconds=30)
+        detected_at = datetime.datetime.now(tz=datetime.UTC)
         app.state.pending_recovery = PendingRecovery(
             jail_name="sshd",
             activated_at=activated_at,