fix: also catch RuntimeError in derive_entry_spot exception handler

Databento can raise RuntimeError for API key issues, but derive_entry_spot
only caught ValueError and KeyError. This ensures Databento errors are
properly caught and displayed to the user.

app/pages/backtests.py

@@ -529,7 +529,7 @@ def _render_backtests_page(workspace_id: str | None = None) -> None:
                     parse_iso_date(end_input.value, "End date"),
                     data_source=str(data_source_select.value),
                 )
-            except (ValueError, KeyError) as exc:
+            except (ValueError, KeyError, RuntimeError) as exc:
                 return None, str(exc)
             return resolved_entry_spot, None