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An API reference and rationale for whoever picks this up next. The endpoints are the easy half; what is not obvious from reading the code is why an ack is only published after the commit, why a duplicate counts as a success, and why is_delta false on a filtered catalogue empties a shop's shelf. Those are written down here because each one costs a shop money when someone changes it without knowing. Covers the GET endpoints in full — request parameters, real response shapes, and the cases that surprise people: values coming back as strings from a Redis hash, a quiet till answering 200 rather than 404, and a bill at another outlet returning 404 even though the reference is valid. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>