Honour forwarded headers only from trusted proxies (closes #259)

A new ClientIP middleware sets request.RemoteAddr to the client's
address before the request is logged. X-Forwarded-For and X-Real-IP
count only when the peer is in TRUSTED_PROXIES, which defaults to the
RFC 1918 ranges. The login rate limit, the session's IP address and
hostname, and the request log all use that address.

Model: opus-5-5

internal/handlers/api.go

@@ -42,25 +42,9 @@ func (hdlr *Handlers) maxBodySize() int64 {
 	return defaultMaxBodySize
 }
 
-// clientIP extracts the connecting client's IP address
-// from the request, checking X-Forwarded-For and
-// X-Real-IP headers before falling back to RemoteAddr.
+// clientIP returns the IP address in request.RemoteAddr, which the
+// ClientIP middleware has set to the client's address.
 func clientIP(request *http.Request) string {
-	if forwarded := request.Header.Get("X-Forwarded-For"); forwarded != "" {
-		// X-Forwarded-For can contain a comma-separated list;
-		// the first entry is the original client.
-		parts := strings.SplitN(forwarded, ",", 2) //nolint:mnd
-		ip := strings.TrimSpace(parts[0])
-
-		if ip != "" {
-			return ip
-		}
-	}
-
-	if realIP := request.Header.Get("X-Real-IP"); realIP != "" {
-		return strings.TrimSpace(realIP)
-	}
-
 	host, _, err := net.SplitHostPort(request.RemoteAddr)
 	if err != nil {
 		return request.RemoteAddr