audit-1 (critical 1): mandatory operator signatures bound to content+deployment

- signing module: canonical content hashes + approval_message
  sha256(cayce-ops:<type>:<deployment_id>:<id>:<content_hash>)
- approve/approve-draft/publisher-approve REQUIRE signature; without
  CAYCE_OPS_OPERATOR_PUBKEY+DEPLOYMENT_ID the daemons refuse to
  execute/send/publish (no degraded mode)
- run-time re-verification at execute/send/publish: unsigned rows (raw SQL
  approval) or edited content revert to pre-approval state, audited as error
- FULL signatures stored on rows (operator_signature, migration 0003) and
  in audit detail — never truncated
- signature failures audited from CLI wrappers too (1f)
- AUTONOMOUS_MODE approval path removed (D13): no non-human approvals

tests: SQL-approved row refused at execute/send/publish; to_addr change
blocks send; foreign deployment/content signatures refused; full-signature
persistence. 65 tests green.

tests/test_models.py

@@ -110,3 +110,35 @@ def test_audit_chain_and_verification(sess):
     first.actor = "tampered"
     ok, broken = verify_audit_chain(sess)
     assert not ok and broken == first.id
+
+
+def test_concurrent_writers_keep_chain_verifiable(db_engine):
+    """Round-2 point 4: serialized writes -> no forked chains."""
+    import threading
+
+    import cayceops.db as appdb
+
+    errors: list[Exception] = []
+
+    def writer(worker: int, n: int):
+        try:
+            for i in range(n):
+                with appdb.session_scope() as s:
+                    audit(s, actor=f"w{worker}", action=f"step{i}", detail={"worker": worker})
+        except Exception as exc:  # noqa: BLE001
+            errors.append(exc)
+
+    threads = [threading.Thread(target=writer, args=(w, 25)) for w in range(4)]
+    for t in threads:
+        t.start()
+    for t in threads:
+        t.join()
+
+    assert not errors, errors
+    s = appdb.session_factory()()
+    total = s.query(AuditEntry).count()
+    assert total == 100
+    from cayceops.models import verify_audit_chain
+
+    ok, broken = verify_audit_chain(s)
+    assert ok and broken is None