fix: extract real client IP from proxy headers (X-Real-IP / X-Forwarded-For)

Behind a reverse proxy like Traefik, RemoteAddr always contains the
proxy's IP. Add realIP() helper that checks X-Real-IP first, then the
first entry of X-Forwarded-For, falling back to RemoteAddr.

Update both LoginRateLimit and Logging middleware to use realIP().
Add comprehensive tests for the new function.

Fixes #12

internal/middleware/middleware.go

@@ -7,6 +7,7 @@ import (
 	"math"
 	"net"
 	"net/http"
+	"strings"
 	"sync"
 	"time"
 
@@ -90,7 +91,7 @@ func (m *Middleware) Logging() func(http.Handler) http.Handler {
 					"request_id", reqID,
 					"referer", request.Referer(),
 					"proto", request.Proto,
-					"remoteIP", ipFromHostPort(request.RemoteAddr),
+					"remoteIP", realIP(request),
 					"status", lrw.statusCode,
 					"latency_ms", latency.Milliseconds(),
 				)
@@ -110,6 +111,28 @@ func ipFromHostPort(hostPort string) string {
 	return host
 }
 
+// realIP extracts the client's real IP address from the request,
+// checking proxy headers first (trusted reverse proxy like Traefik),
+// then falling back to RemoteAddr.
+func realIP(r *http.Request) string {
+	// 1. X-Real-IP (set by Traefik/nginx)
+	if ip := strings.TrimSpace(r.Header.Get("X-Real-IP")); ip != "" {
+		return ip
+	}
+
+	// 2. X-Forwarded-For: take the first (leftmost/client) IP
+	if xff := r.Header.Get("X-Forwarded-For"); xff != "" {
+		if parts := strings.SplitN(xff, ",", 2); len(parts) > 0 { //nolint:mnd
+			if ip := strings.TrimSpace(parts[0]); ip != "" {
+				return ip
+			}
+		}
+	}
+
+	// 3. Fall back to RemoteAddr
+	return ipFromHostPort(r.RemoteAddr)
+}
+
 // CORS returns CORS middleware.
 func (m *Middleware) CORS() func(http.Handler) http.Handler {
 	return cors.Handler(cors.Options{
@@ -241,7 +264,7 @@ func (m *Middleware) LoginRateLimit() func(http.Handler) http.Handler {
 			writer http.ResponseWriter,
 			request *http.Request,
 		) {
-			ip := ipFromHostPort(request.RemoteAddr)
+			ip := realIP(request)
 			limiter := loginLimiter.getLimiter(ip)
 
 			if !limiter.Allow() {