Split internal/web into subpackages before domain-admin growth.

Lay out view, auth, validate, and handlers under internal/web while keeping
the cmd/panel API unchanged; update roadmap and changelog for web-split closure.

Co-authored-by: Cursor <cursoragent@cursor.com>

internal/web/handlers/testutil_test.go

@@ -0,0 +1,16 @@
+package handlers
+
+import (
+	"testing"
+
+	"github.com/mixeme/selfpost/internal/web/view"
+)
+
+func mustView(t *testing.T) *view.Engine {
+	t.Helper()
+	v, err := view.New("test")
+	if err != nil {
+		t.Fatalf("view: %v", err)
+	}
+	return v
+}