feat: use resolved client IP in request and auth logs

Add a ClientIP middleware that resolves the client address once per
request and stores it in the context, placed ahead of logging in the
route chain. The request-logging middleware and both login-attempt logs
now read the resolved address instead of the raw peer. Behind a trusted
proxy these records show the real client; a direct, untrusted client
still shows its own address and cannot forge one. Removes the local
host:port helper now that the clientip package owns that parsing.

Model: opus-4-8

internal/handlers/auth.go

@@ -8,6 +8,7 @@ import (
 	"strconv"
 	"time"
 
+	"sneak.berlin/go/pixa/internal/clientip"
 	"sneak.berlin/go/pixa/internal/encurl"
 	"sneak.berlin/go/pixa/internal/imgcache"
 	"sneak.berlin/go/pixa/internal/templates"
@@ -47,7 +48,8 @@ func (s *Handlers) handleLoginPost(w http.ResponseWriter, r *http.Request) {
 
 	// Constant-time comparison to prevent timing attacks
 	if subtle.ConstantTimeCompare([]byte(submittedKey), []byte(s.config.SigningKey)) != 1 {
-		s.log.Warn("failed login attempt", "remote_addr", r.RemoteAddr)
+		s.log.Warn("failed login attempt",
+			"remote_addr", clientip.FromContext(r.Context()))
 		s.renderLogin(w, r, "Invalid signing key")
 
 		return
@@ -62,7 +64,8 @@ func (s *Handlers) handleLoginPost(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	s.log.Info("successful login", "remote_addr", r.RemoteAddr)
+	s.log.Info("successful login",
+		"remote_addr", clientip.FromContext(r.Context()))
 
 	// Redirect to generator page
 	http.Redirect(w, r, "/", http.StatusSeeOther)