Percent-decode URL parameters before parsing them (closes #103)

chi matches routes against the escaped path when a request has one and
hands the handlers their URL parameters still percent-encoded. The home
page's lookup form encodes the address it asks for, so every IPv6 lookup
answered 400, as did a comma-separated address list or a prefix that a
client percent-encoded. The router now matches against the decoded path,
so every handler reads its parameters decoded.

Model: opus-5-5

internal/server/routes.go

@@ -1,6 +1,7 @@
 package server
 
 import (
+	"net/http"
 	"time"
 
 	"github.com/go-chi/chi/v5"
@@ -12,6 +13,18 @@ func (s *Server) setupRoutes() {
 	r := chi.NewRouter()
 
 	// Middleware
+
+	// Match routes against the decoded path, so that the handlers read their
+	// URL parameters decoded. chi otherwise matches against the escaped path
+	// when the request has one and leaves the parameters escaped:
+	// /ip/2001%3Adb8%3A%3A1 would look up "2001%3Adb8%3A%3A1".
+	r.Use(func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
+			chi.RouteContext(req.Context()).RoutePath = req.URL.Path
+
+			next.ServeHTTP(w, req)
+		})
+	})
 	r.Use(middleware.RequestID)
 	r.Use(middleware.RealIP)
 	r.Use(RequestLoggerMiddleware(s.logger.Logger)) // Structured request logging