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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backend/app/services/blocklist_service.py

@@ -480,7 +480,13 @@ async def get_schedule_info(
     config = await get_schedule(db)
     last_log = await import_log_repo.get_last_log(db)
     last_run_at = last_log["timestamp"] if last_log else None
-    return ScheduleInfo(config=config, next_run_at=next_run_at, last_run_at=last_run_at)
+    last_run_errors: bool | None = (last_log["errors"] is not None) if last_log else None
+    return ScheduleInfo(
+        config=config,
+        next_run_at=next_run_at,
+        last_run_at=last_run_at,
+        last_run_errors=last_run_errors,
+    )
 
 
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