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/internal/backup/shares_restore.go

@@ -5,9 +5,9 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	"gitea.dooplex.hu/admin/felhom-controller/internal/dockerexec"
 	"gitea.dooplex.hu/admin/felhom-controller/internal/util"
 	"os"
-	"os/exec"
 	"path/filepath"
 	"strings"
 
@@ -45,7 +45,7 @@ func (m *Manager) sharesPassdbRestorer() func([]byte) error {
 // writes ONLY into infra.SambaPassdbMount inside the samba container — never onto the host — so a
 // malformed archive cannot reach anything outside the volume it came from.
 func defaultSharesPassdbRestore(tar []byte) error {
-	cmd := exec.Command("docker", "exec", "-i", infra.SambaContainerName,
+	cmd := dockerexec.Command("docker", "exec", "-i", infra.SambaContainerName,
 		"tar", "xf", "-", "-C", infra.SambaPassdbMount)
 	cmd.Stdin = bytes.NewReader(tar)
 	out, err := cmd.CombinedOutput()