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/web/r650_main_test.go

@@ -0,0 +1,12 @@
+package web
+
+import (
+	"os"
+	"testing"
+
+	"gitea.dooplex.hu/admin/felhom-controller/internal/dockerexec"
+)
+
+// TestMain puts a silent docker stub on PATH for every test in this package: R-650, the fixtures
+// here build a real stacks.Manager, and on the build host (DooPlex) that reached production Docker.
+func TestMain(m *testing.M) { os.Exit(dockerexec.RunWithStub(m)) }