SIENTIAPDE-1975: the gateway is the only writer that updates write_status

The Laborious no longer records OPC outcomes (it inserts the row as
'pending' and only publishes the commands), so the comments and docs that
asked to keep a Laborious copy of the UPDATE in sync are updated.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -91,8 +91,9 @@ The Laborious publishes one command per tag (SIENTIAPDE-1975):
   Lookups are cached for 60 s. An unknown server is a failed write (`server_not_found`).
 - **Outcome**: `UPDATE ... SET write_status` on the prediction row (by `timestamp` +
   `model_id`), keeping the worst outcome: `ok` < `error` < `session_bad`, and
-  `pending`/NULL are replaced by any answer. The Laborious applies the same rule to
-  the outcomes it records itself, so the order of the answers does not matter.
+  `pending`/NULL are replaced by any answer, so the order of the answers does not
+  matter. The gateway is the only one that updates the column: the Laborious inserts the
+  row as `pending` (merged with its PI Web API outcome) and never records OPC outcomes.
   The UPDATE never raises; its outcome (`updated`, `kept`, `row_not_found`,
   `invalid`, `error`) is logged and counted in `opc_write_status_update_total`.
   The reason of a failed write is logged by `WriteEngine`.