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).

frontend/src/hooks/useBlocklist.ts

@@ -235,3 +235,51 @@ export function useRunImport(): UseRunImportReturn {
 
   return { running, lastResult, error, runNow };
 }
+
+// ---------------------------------------------------------------------------
+// useBlocklistStatus
+// ---------------------------------------------------------------------------
+
+/** How often to re-check the schedule endpoint for import errors (ms). */
+const BLOCKLIST_POLL_INTERVAL_MS = 60_000;
+
+export interface UseBlocklistStatusReturn {
+  /** `true` when the most recent import run completed with errors. */
+  hasErrors: boolean;
+}
+
+/**
+ * Poll `GET /api/blocklists/schedule` every 60 seconds to detect whether
+ * the most recent blocklist import had errors.
+ *
+ * Network failures during polling are silently ignored — the indicator
+ * simply retains its previous value until the next successful poll.
+ */
+export function useBlocklistStatus(): UseBlocklistStatusReturn {
+  const [hasErrors, setHasErrors] = useState(false);
+
+  useEffect(() => {
+    let cancelled = false;
+
+    const poll = (): void => {
+      fetchSchedule()
+        .then((info) => {
+          if (!cancelled) {
+            setHasErrors(info.last_run_errors === true);
+          }
+        })
+        .catch(() => {
+          // Silently swallow network errors — do not change indicator state.
+        });
+    };
+
+    poll();
+    const id = window.setInterval(poll, BLOCKLIST_POLL_INTERVAL_MS);
+    return (): void => {
+      cancelled = true;
+      window.clearInterval(id);
+    };
+  }, []);
+
+  return { hasErrors };
+}