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/types/blocklist.ts

@@ -71,6 +71,8 @@ export interface ScheduleInfo {
   config: ScheduleConfig;
   next_run_at: string | null;
   last_run_at: string | null;
+  /** `true` if the most recent import had errors, `false` if clean, `null` if never run. */
+  last_run_errors: boolean | null;
 }
 
 // ---------------------------------------------------------------------------