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/exceptions.py

@@ -503,6 +503,19 @@ class BlocklistSourceHasLogsError(ConflictError):
         return {"source_id": self.source_id}
 
 
+class BlocklistSourceAlreadyExistsError(ConflictError):
+    """Raised when a blocklist source with the same URL already exists."""
+
+    error_code: str = "blocklist_source_already_exists"
+
+    def __init__(self, url: str) -> None:
+        self.url = url
+        super().__init__(f"Blocklist source with URL already exists: {url}")
+
+    def get_error_metadata(self) -> ErrorMetadata:
+        return {"url": self.url}
+
+
 class HistoryNotFoundError(NotFoundError):
     """Raised when no history is found for the given IP."""