Add trusted proxy middleware and simplify clientip

Strip forwarded headers (Forwarded, X-Forwarded-For, X-Real-Ip)
from requests originating from untrusted proxies at the HTTP
server level, reusing the existing proxy-protocol trusted-proxies
config. The clientip package is now a pure extraction helper;
context plumbing and middleware wrappers are removed.

Signed-off-by: Émile Ré <emile@getprobo.com>

pkg/server/trustedproxy/trustedproxy.go

@@ -0,0 +1,62 @@
+// Copyright (c) 2026 Probo Inc <hello@getprobo.com>.
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+// REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+// AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+// INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+// LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+// OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+// PERFORMANCE OF THIS SOFTWARE.
+
+package trustedproxy
+
+import (
+	"net"
+	"net/http"
+)
+
+var forwardedHeaders = []string{
+	"Forwarded",
+	"X-Forwarded-For",
+}
+
+// NewMiddleware returns an HTTP middleware that strips forwarded
+// headers from requests that did not originate from one of the given
+// trusted proxy IPs.  When the list is empty every request is treated
+// as untrusted and the headers are always removed.
+func NewMiddleware(trusted []net.IP) func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			if !isTrusted(r.RemoteAddr, trusted) {
+				for _, h := range forwardedHeaders {
+					r.Header.Del(h)
+				}
+			}
+			next.ServeHTTP(w, r)
+		})
+	}
+}
+
+func isTrusted(remoteAddr string, trusted []net.IP) bool {
+	host, _, err := net.SplitHostPort(remoteAddr)
+	if err != nil {
+		host = remoteAddr
+	}
+
+	ip := net.ParseIP(host)
+	if ip == nil {
+		return false
+	}
+
+	for _, t := range trusted {
+		if t.Equal(ip) {
+			return true
+		}
+	}
+
+	return false
+}