The go-sdk Streamable HTTP handler enables DNS-rebinding protection that
rejects any request whose Host header is non-loopback while the server is
bound to a loopback address. memos is commonly run bound to loopback behind a
reverse proxy (e.g. the public demo), so every /mcp request was rejected with
"403 Forbidden: invalid Host header" before authentication ran.
Disable the SDK's localhost protection and rely on memos' own Origin/Host
allowlist (isAllowedMCPOrigin) for CSRF / DNS-rebinding protection. Add a
regression test covering the proxied shape and confirming disallowed origins
are still rejected.
