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>

backend/app/main.py

@@ -182,6 +182,7 @@ async def _lifespan(app: FastAPI) -> AsyncGenerator[None, None]:
     global _external_log_handler  # noqa: PLW0603
 
     settings: Settings = app.state.settings
+    runtime_state = app.state.runtime_state
 
     http_session, scheduler, startup_db = await startup_shared_resources(app, settings)
     app.state.http_session = http_session
@@ -210,10 +211,18 @@ async def _lifespan(app: FastAPI) -> AsyncGenerator[None, None]:
             if _external_log_handler:
                 _external_log_handler.start_periodic_flush()
         except ValueError as exc:
-            log.warning(
+            from app.utils import metrics as _metrics_mod
+
+            _metrics_mod.external_logging_init_failures.inc()
+            runtime_state.external_log_init_failed = True
+            log.error(
                 "external_logging_initialization_failed",
                 error=str(exc),
             )
+            if settings.external_log_required:
+                msg = f"External logging is required but handler creation failed: {exc}"
+                log.critical("external_logging_required_but_unavailable", error=str(exc))
+                raise RuntimeError(msg) from exc
 
     # Now configure logging with the handler in place
     _configure_logging(settings.log_level, settings)