fix: retry execution attempt number conflicts

Co-authored-by: Copilot App <[email protected]>

docs/production-runbook.md

@@ -141,3 +141,13 @@ and exit cleanly after the call returns.
 Set the container or service termination grace period **above this total**
 budget. If termination grace is shorter, the process may be killed before
 terminal status and evidence writes are finalized.
+
+## 9. Horizontal scaling precondition
+
+Multiple worker replicas can race on execution-attempt numbering for the same
+`(job_id, source_id)` pair. The runtime now retries boundedly on unique-key
+conflicts (`uq_execution_attempt_number`) and surfaces a conflict-domain error
+if retries are exhausted.
+
+Do not deploy additional worker replicas unless this conflict-retry path and its
+tests are present and green in the target build.