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/config.py

@@ -450,6 +450,16 @@ class Settings(BaseSettings):
             "Logs are sent earlier if the batch size is reached."
         ),
     )
+    external_log_required: bool = Field(
+        default=False,
+        description=(
+            "When enabled and external logging is configured, startup aborts if the "
+            "external log handler fails to initialize. When disabled (default), a failed "
+            "handler is treated as a warning and the application continues without external "
+            "logging. Set to true in production environments where logs must reach the "
+            "monitoring system."
+        ),
+    )
     datadog_api_key: str | None = Field(
         default=None,
         description=(