How to get a free API key

The key endpoint is POST-only; a GET used to answer with a bare 405.

Both observed /keys visitors arrived without converting, and a 405 tells an exploring developer nothing. Answer with the how-to instead. In the schema (not include_in_schema=False) because the consumers the CTA headers point here include OpenAPI-driven tooling — consumer B generated MCP tools from our spec and would otherwise be told to call a route the spec denies exists. Rate-limited like every other /api/v1 GET so the 429 hint cannot send callers into an uncapped retry loop.