#923 datashard hardening before upgrade: narrow _get_or_create_table, unify storage default, fatal on missing S3 creds

EARS SPEC: - If loading an existing datashard table fails for any reason other than not-found, then the logger shall raise, not create a new table. - All modules shall read one storage_type default. - If storage_type=s3 and credentials are missing, then the service shall fail to start. Parent epic: #913. Synthesis (technical problems / solution domains / alternatives) recorded as a comment when started.