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/middleware.go

@@ -10,6 +10,8 @@ import (
 	"net/http"
 	"sync"
 	"time"
+
+	"github.com/go-chi/chi/v5"
 )
 
 // responseWriter wraps http.ResponseWriter to capture the response
@@ -281,3 +283,16 @@ func RequestLoggerMiddleware(logger *slog.Logger) func(http.Handler) http.Handle
 		})
 	}
 }
+
+// DecodedPathMiddleware makes the router match routes against the decoded
+// path, so that the handlers read their URL parameters decoded. Without it,
+// chi matches against the escaped path whenever the request has one and hands
+// the handlers their parameters still escaped: /ip/2001%3Adb8%3A%3A1 would
+// look up "2001%3Adb8%3A%3A1".
+func DecodedPathMiddleware(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		chi.RouteContext(r.Context()).RoutePath = r.URL.Path
+
+		next.ServeHTTP(w, r)
+	})
+}