Run immediate health probe after jail deactivation

After deactivation the endpoint now calls _run_probe to flush the
cached server status immediately, matching the activate_jail behaviour
added in Task 5. Without this, the dashboard active-jail count could
remain stale for up to 30 s after a deactivation reload.

- config.py: capture result, await _run_probe, return result
- test_config.py: add test_deactivate_triggers_health_probe; fix 3
  pre-existing UP017 ruff warnings (datetime.UTC alias)
- test_health.py: update test to assert the new fail2ban field

backend/tests/test_routers/test_config.py

@@ -847,6 +847,30 @@ class TestDeactivateJail:
         ).post("/api/config/jails/sshd/deactivate")
         assert resp.status_code == 401
 
+    async def test_deactivate_triggers_health_probe(self, config_client: AsyncClient) -> None:
+        """POST .../deactivate triggers an immediate health probe after success."""
+        from app.models.config import JailActivationResponse
+
+        mock_response = JailActivationResponse(
+            name="sshd",
+            active=False,
+            message="Jail 'sshd' deactivated successfully.",
+        )
+        with (
+            patch(
+                "app.routers.config.config_file_service.deactivate_jail",
+                AsyncMock(return_value=mock_response),
+            ),
+            patch(
+                "app.routers.config._run_probe",
+                AsyncMock(),
+            ) as mock_probe,
+        ):
+            resp = await config_client.post("/api/config/jails/sshd/deactivate")
+
+        assert resp.status_code == 200
+        mock_probe.assert_awaited_once()
+
 
 # ---------------------------------------------------------------------------
 # GET /api/config/filters
@@ -1992,7 +2016,7 @@ class TestPendingRecovery:
 
         from app.models.config import PendingRecovery
 
-        now = datetime.datetime.now(tz=datetime.timezone.utc)
+        now = datetime.datetime.now(tz=datetime.UTC)
         record = PendingRecovery(
             jail_name="sshd",
             activated_at=now - datetime.timedelta(seconds=20),
@@ -2031,7 +2055,7 @@ class TestRollbackEndpoint:
 
         # Set up a pending recovery record on the app.
         app = config_client._transport.app  # type: ignore[attr-defined]
-        now = datetime.datetime.now(tz=datetime.timezone.utc)
+        now = datetime.datetime.now(tz=datetime.UTC)
         app.state.pending_recovery = PendingRecovery(
             jail_name="sshd",
             activated_at=now - datetime.timedelta(seconds=10),
@@ -2067,7 +2091,7 @@ class TestRollbackEndpoint:
         from app.models.config import PendingRecovery, RollbackResponse
 
         app = config_client._transport.app  # type: ignore[attr-defined]
-        now = datetime.datetime.now(tz=datetime.timezone.utc)
+        now = datetime.datetime.now(tz=datetime.UTC)
         record = PendingRecovery(
             jail_name="sshd",
             activated_at=now - datetime.timedelta(seconds=10),