Fix http transport rejecting remote clients with 421#4
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FastMCP is constructed at import time with the default host 127.0.0.1, which auto-locks DNS-rebinding protection to a localhost-only Host allowlist. Rebinding the host to 0.0.0.0 in main() does not update that frozen allowlist, so remote clients get "421 Invalid Host header". Set transport_security explicitly in http mode: derive allowed hosts from VYMCP_PUBLIC_URL (overridable via VYMCP_ALLOWED_HOSTS, or "*" to disable the check behind a trusted proxy).
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FastMCP is constructed at import time with the default host 127.0.0.1, which auto-locks DNS-rebinding protection to a localhost-only Host allowlist. Rebinding the host to 0.0.0.0 in main() does not update that frozen allowlist, so remote clients get "421 Invalid Host header".
Set transport_security explicitly in http mode: derive allowed hosts from VYMCP_PUBLIC_URL (overridable via VYMCP_ALLOWED_HOSTS, or "*" to disable the check behind a trusted proxy).