docs: document change_state and list_states endpoints

[felipegarcia98]

Why

change_state and list_states have been live in the external API for a long time, and they are what the Change Form Status action of the Zapier and Power Automate connectors calls, but neither was in this reference. Anyone integrating without those connectors had no way to find them.

What changed

Two new subsections in Answers, right after Change Answer:

• Get Answer Statuses (GET/POST /api/external/list_states) — no parameters, returns type / key / identifier, plus a note that a status deleted in the web app is still listed because the response is not filtered by the deleted flag (the Answers endpoints report - for those instead).
• Change Answer Status (POST/GET /api/external/change_state) — form_name, form_code and form_state_id, all required. Documents that form_name accepts the form's name only and not its ID, that the most recent form wins when two share a name, the side effects of the change (history entry, status-change webhooks, push notification when the account has it enabled), and that a 400 comes back with an empty body for any of the three failed lookups.

Plus the 10-Sep-2026 changelog entry.

Base is main, since that is where the documentation is maintained.

Notes

• Markdown only, 137 lines added, nothing removed. Reuses the headings already in the file (### HTTP Request, ### Input Parameter, ### Response Fields, ### Return Codes).
• Not built locally: the repo pins Ruby 2.3.3 and it is not installed on this machine. The Build workflow covers it.
• Out of scope, spotted while reading the connector definition: apiDefinition.swagger.json declares key on /list_states as string, while the backend returns an integer. Left untouched.

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