MCP event stream (streamable-HTTP transport)

Proxied verbatim to Kairos. The gateway applies NO authentication of its own here — /v1 and /mcp sit outside the /v2 middleware, so the Authorization header is forwarded untouched and Kairos is the sole authority. In practice that means either il_* credential is accepted (an API key, or an il_oauth_ token — the latter is how an MCP client reaches /mcp); a Supabase session JWT is NOT, since Kairos cannot verify one. Response shapes are owned by Kairos. The gateway forwards ALL methods under /v1/* and /mcp — the operations listed here are the primary surface, not an exhaustive proxy inventory.