Fix #280 follow-up: remove PALETTE_TRUSTED_IP_HEADER

Per owner decision the optional proxy-header escape hatch is dead config:
remove the env var, its plumbing (Config.TrustedIPHeader, SetTrustedIPHeader),
and the README row. Rate-limit keying is always the peer address; no
client-supplied IP header is ever trusted. Tests updated to assert headers
(CF-Connecting-IP included) never influence clientIP.

internal/api/clientip.go

@@ -7,48 +7,18 @@
 // per request and the limit was unenforceable (pentest H1: 6x201 across 8
 // rotating-XFF creates).
 //
-// Default: key on the actual peer address (RemoteAddr) only. Behind any
+// The bucket key is always the actual peer address (RemoteAddr). Behind any
 // reverse proxy this is the proxy's address, so all clients share one bucket
-// per endpoint — coarse, but safe.
-//
-// Proxy-honoring mode: a deployment in front of a proxy that OVERWRITES (not
-// appends to) a client-IP header can set PALETTE_TRUSTED_IP_HEADER (e.g.
-// CF-Connecting-IP when Cloudflare is the ingress; Cloudflare strips any
-// client-supplied value). The header is honored ONLY when explicitly
-// configured at startup, and X-Forwarded-For / X-Real-Ip are never trusted.
+// per endpoint — coarse, but safe. Client-supplied IP headers
+// (X-Forwarded-For, X-Real-Ip, and any others) are never trusted.
 package api
 
 import (
 	"net"
 	"net/http"
-	"sync"
 )
 
-var (
-	trustedIPMu     sync.RWMutex
-	trustedIPHeader string // empty = never trust any client-IP header
-)
-
-// SetTrustedIPHeader configures the single proxy-controlled header whose
-// value may key rate-limit buckets. Called at startup; tests may reset it.
-func SetTrustedIPHeader(name string) {
-	trustedIPMu.Lock()
-	defer trustedIPMu.Unlock()
-	trustedIPHeader = name
-}
-
-func getTrustedIPHeader() string {
-	trustedIPMu.RLock()
-	defer trustedIPMu.RUnlock()
-	return trustedIPHeader
-}
-
 func clientIP(r *http.Request) string {
-	if name := getTrustedIPHeader(); name != "" {
-		if v := r.Header.Get(name); v != "" {
-			return v
-		}
-	}
 	host := r.RemoteAddr
 	if h, _, err := net.SplitHostPort(r.RemoteAddr); err == nil {
 		host = h