Show blocklist import error badge in navigation

When the most recent scheduled import completed with errors, surface the
failure in the persistent app shell:
- A warning MessageBar appears at top of main content area
- An amber badge is rendered on the Blocklists sidebar nav item

Backend: add last_run_errors: bool | None to ScheduleInfo model and
populate it in get_schedule_info() from the latest import_log row.

Frontend: extend ScheduleInfo type, add useBlocklistStatus polling hook,
wire both indicators into MainLayout.

Tests: 3 new service tests + 1 new router test (433 total, all pass).

backend/tests/test_routers/test_blocklist.py

@@ -379,6 +379,31 @@ class TestGetSchedule:
         assert "next_run_at" in body
         assert "last_run_at" in body
 
+    async def test_schedule_response_includes_last_run_errors(
+        self, bl_client: AsyncClient
+    ) -> None:
+        """GET /api/blocklists/schedule includes last_run_errors field."""
+        info_with_errors = ScheduleInfo(
+            config=ScheduleConfig(
+                frequency=ScheduleFrequency.daily,
+                interval_hours=24,
+                hour=3,
+                minute=0,
+                day_of_week=0,
+            ),
+            next_run_at=None,
+            last_run_at="2026-03-01T03:00:00+00:00",
+            last_run_errors=True,
+        )
+        with patch(
+            "app.routers.blocklist.blocklist_service.get_schedule_info",
+            new=AsyncMock(return_value=info_with_errors),
+        ):
+            resp = await bl_client.get("/api/blocklists/schedule")
+        body = resp.json()
+        assert "last_run_errors" in body
+        assert body["last_run_errors"] is True
+
 
 # ---------------------------------------------------------------------------
 # PUT /api/blocklists/schedule