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>

Docs/Deployment.md

@@ -97,6 +97,7 @@ monitoring integration:
 | database | Opens and closes a test connection | Returns degraded when failing |
 | scheduler | `scheduler.running` attribute | Returns degraded when stopped |
 | cache | Session cache presence | Returns degraded when not initialised |
+| external_logging | Handler initialization status | Returns degraded when failed |
 
 ### Kubernetes Probes — Liveness and Readiness