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/app/models/blocklist.py

@@ -133,6 +133,8 @@ class ScheduleInfo(BaseModel):
     config: ScheduleConfig
     next_run_at: str | None
     last_run_at: str | None
+    last_run_errors: bool | None = None
+    """``True`` if the most recent import had errors, ``False`` if clean, ``None`` if never run."""
 
 
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