create_decision_log now checks for an existing log with the same
(eval_id, decision_type, context) tuple before inserting. If found, it
returns the existing row's dict instead of appending a duplicate. The
append-only audit invariant is preserved (a worker that re-emits the
same decision within a single minute no longer produces duplicate rows).
Removed the xfail guard in test_decision_log_immutability; the test
now passes (3 identical POSTs → 1 DB row).
4bcab065f1