Fix rate limiter bypass via client-controlled X-Forwarded-For (#280)

clientIP() keyed rate-limit buckets on the rightmost X-Forwarded-For
entry, assuming traefik appends the real client IP. The deployed ingress
does not rewrite XFF, so rotating the header gave a fresh bucket per
request (pentest H1: 8 creates with rotating XFF -> 6x201).

Now the bucket keys on the actual peer address (RemoteAddr) by default;
every client-supplied IP header is ignored. Deployments whose ingress
overwrites a client-IP header can opt in via PALETTE_TRUSTED_IP_HEADER
(e.g. CF-Connecting-IP behind Cloudflare) to restore per-client limits.

Adds tests: rotating XFF no longer resets the bucket; the trusted header
is honored only when explicitly configured.

internal/api/ratelimit.go

@@ -3,7 +3,6 @@ package api
 import (
 	"net/http"
 	"strconv"
-	"strings"
 	"sync"
 	"time"
 )
@@ -48,35 +47,6 @@ func (l *limiter) allow(key string, rate, burst float64) bool {
 	return true
 }
 
-// clientIP extracts the client IP for rate-limit keying (#85).
-//
-// Trust boundary: palette runs behind exactly ONE trusted reverse proxy
-// (Traefik in the k3s pod network). Traefik APPENDS the real client IP to
-// X-Forwarded-For, so the RIGHTMOST entry is the last value the trusted
-// proxy observed and is unspoofable by the client (a client-supplied fake
-// entry only lands on the LEFT and is ignored). This matches chi's
-// middleware.RealIP semantics for a single trusted proxy hop.
-//
-// Direct connections (no XFF header) fall back to RemoteAddr. Directly
-// reachable deployments must NOT expose the app to untrusted networks
-// without a proxy in front, or attackers could forge the rightmost entry.
-func clientIP(r *http.Request) string {
-	if xff := r.Header.Get("X-Forwarded-For"); xff != "" {
-		if i := strings.LastIndex(xff, ","); i >= 0 {
-			return strings.TrimSpace(xff[i+1:])
-		}
-		return strings.TrimSpace(xff)
-	}
-	if xr := r.Header.Get("X-Real-Ip"); xr != "" {
-		return strings.TrimSpace(xr)
-	}
-	host := r.RemoteAddr
-	if i := strings.LastIndex(host, ":"); i > 0 {
-		host = host[:i]
-	}
-	return host
-}
-
 var globalLimiter = newLimiter()
 
 // globalSettingsFn is set at startup; tests can point it at fixed settings.