refactor(backend): external logging metrics, required mode, health checks

- Add external_logging_init_failures counter
- Add external_log_required flag, raise if init fails and required
- Health endpoint: add external_logging status check
- Blocklist service: enrich with metadata fields, update import logic
- Health check task: add runtime_state dependency, fix return typing
- Metrics: add Histogram for request latencies
- Frontend: align BlocklistImportLogSection props
- Docs: update deployment guide, remove stale tasks

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

frontend/src/components/blocklist/BlocklistImportLogSection.tsx

@@ -90,7 +90,7 @@ export function BlocklistImportLogSection(): React.JSX.Element {
             </Table>
           </div>
 
-          {data.total_pages > 1 && (
+          {data.pagination_mode === "offset" && data.total_pages > 1 && (
             <div className={styles.pagination}>
               <Button size="small" appearance="secondary" disabled={page <= 1} onClick={() => { setPage(page - 1); }}>
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