Resolve real client IP behind trusted proxies (closes #94)

RFC1918 ranges are the default trusted proxy set on an omitted key; an explicit list replaces the default; an explicit empty list trusts no one; unparseable values abort startup; forwarded headers honored only from trusted peers. Independent review passed: #127 (comment)

model: claude-opus-4-8 (implementation and review); merged by claude-fable-5

internal/middleware/middleware.go

@@ -3,7 +3,6 @@ package middleware
 
 import (
 	"log/slog"
-	"net"
 	"net/http"
 	"time"
 
@@ -14,6 +13,7 @@ import (
 	ghmm "github.com/slok/go-http-metrics/middleware"
 	"github.com/slok/go-http-metrics/middleware/std"
 	"go.uber.org/fx"
+	"sneak.berlin/go/pixa/internal/clientip"
 	"sneak.berlin/go/pixa/internal/config"
 	"sneak.berlin/go/pixa/internal/logger"
 )
@@ -58,31 +58,34 @@ type Params struct {
 
 // Middleware provides HTTP middleware functions.
 type Middleware struct {
-	log    *slog.Logger
-	config *config.Config
+	log      *slog.Logger
+	config   *config.Config
+	clientIP *clientip.Resolver
 }
 
 // New creates a new Middleware instance.
 func New(_ fx.Lifecycle, params Params) (*Middleware, error) {
 	s := &Middleware{
-		log:    params.Logger.Get(),
-		config: params.Config,
+		log:      params.Logger.Get(),
+		config:   params.Config,
+		clientIP: clientip.NewResolver(params.Config.TrustedProxies),
 	}
 
 	return s, nil
 }
 
-func ipFromHostPort(hp string) string {
-	h, _, err := net.SplitHostPort(hp)
-	if err != nil {
-		return ""
+// ClientIP returns a middleware that resolves the real client IP,
+// honoring X-Forwarded-For only from trusted proxies, and stores it in
+// the request context for the logging middleware and handlers to read.
+func (s *Middleware) ClientIP() func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			ip := s.clientIP.Resolve(
+				r.RemoteAddr, r.Header.Values(clientip.ForwardedForHeader))
+			ctx := clientip.WithClientIP(r.Context(), ip)
+			next.ServeHTTP(w, r.WithContext(ctx))
+		})
 	}
-
-	if len(h) > 0 && h[0] == '[' {
-		return h[1 : len(h)-1]
-	}
-
-	return h
 }
 
 type loggingResponseWriter struct {
@@ -127,7 +130,7 @@ func (s *Middleware) Logging() func(http.Handler) http.Handler {
 					"request_id", reqID,
 					"referer", r.Referer(),
 					"proto", r.Proto,
-					"remoteIP", ipFromHostPort(r.RemoteAddr),
+					"remoteIP", clientip.FromContext(ctx),
 					"status", lrw.statusCode,
 					"response_bytes", lrw.bytesWritten,
 					"latency_ms", latency.Milliseconds(),