Adds SanitizeHTTPURL / SanitizePublicHTTPURL to reject private-range
and loopback targets before any outbound HTTP request (node probe,
xray download, outbound test, external traffic inform, tgbot API
server, panel updater). Forwarded headers (X-Real-IP, X-Forwarded-For,
X-Forwarded-Host) are now only trusted when the direct connection
arrives from a CIDR in TrustedProxyCIDRs. CSP policy is tightened with
a per-request nonce. HTTP server gains read/write/idle timeouts. Panel
updater downloads the script to a temp file instead of piping curl into
shell. Xray archive download adds a size cap and response-code check.
backuptotgbot is changed from GET to POST.
