controller v0.267.0: tests off DooPlex's Docker, cut-off copies refused, two pages true

R-650: internal/dockerexec — every docker exec routed through it; under
go test a real docker is refused (opt-in FELHOM_TEST_REAL_DOCKER=1; a stub
under the temp dir is allowed). api/stacks/web tests run under a silent
stub (TestMain). TestR650_NoBareDockerExec pins it repo-wide.
R-640: a dump without its engine's completion marker is refused before
the first mutation (unit + off-site restore) and again before any load.
R-499: the Tier-2 page's system-disk sentence has four true branches.
R-518: the backup button states the measured ~8 min stop.
R-626: measured on 9202, not reproduced.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0159rPz1ZhFKsS53msqPYxtS

controller/cmd/controller/main.go

@@ -6,11 +6,11 @@ import (
 	"errors"
 	"flag"
 	"fmt"
+	"gitea.dooplex.hu/admin/felhom-controller/internal/dockerexec"
 	"io"
 	"log"
 	"net/http"
 	"os"
-	"os/exec"
 	"os/signal"
 	"path/filepath"
 	"sort"
@@ -3019,7 +3019,7 @@ func (a *exportAdapter) RemoveStackVolumes(name string) error {
 
 	ctx, cancel := context.WithTimeout(context.Background(), 2*time.Minute)
 	defer cancel()
-	cmd := exec.CommandContext(ctx, "docker", "compose", "down", "--volumes")
+	cmd := dockerexec.CommandContext(ctx, "docker", "compose", "down", "--volumes")
 	cmd.Dir = stackDir
 	cmd.Env = cmdEnv
 	out, err := cmd.CombinedOutput()