M16: isolate acceptance and harden readiness

backend/tests/test_return.py

@@ -305,3 +305,11 @@ def test_concurrent_returns_only_one_succeeds():
 
     assert results.count(201) == 1
     assert results.count(409) == 2
+
+
+def test_return_public_refs_are_collision_resistant() -> None:
+    from app.services.returns import _new_inspection_ref
+
+    refs = {_new_inspection_ref() for _ in range(1000)}
+    assert len(refs) == 1000
+    assert all(ref.startswith("INSP-") and len(ref) == 15 for ref in refs)